Same problem here
Choppy video fullscreen on youtube, googlevideo, break.com, everywhere :-(

The procedure
---------------------
sudo mkdir /etc/adobe
echo "OverrideGPUValidation=true" >~/mms.cfg
sudo mv ~/mms.cfg /etc/adobe/
------------------------

did NOT solve any problems for me

Until Ubuntu and GNU/Linux doesn't solve these horrible issues, it has zero chance becoming the next desktop OS.
Everything should work out of the box - you can't expect the next door guy to tamper with kernels and stuff from the command line every time they want their system to perform basic stuff.

Here the config of my 2 year old system

Description:	Ubuntu 9.04
Release:	9.04

~$ uname -a

Linux 2.6.28-11-generic #42-Ubuntu SMP Fri Apr 17 01:57:59 UTC 2009 i686 GNU/Linux

~$ lspci

00:00.0 RAM memory: nVidia Corporation MCP61 Memory Controller (rev a1)
00:01.0 ISA bridge: nVidia Corporation MCP61 LPC Bridge (rev a2)
00:01.1 SMBus: nVidia Corporation MCP61 SMBus (rev a2)
00:01.2 RAM memory: nVidia Corporation MCP61 Memory Controller (rev a2)
00:02.0 USB Controller: nVidia Corporation MCP61 USB Controller (rev a2)
00:02.1 USB Controller: nVidia Corporation MCP61 USB Controller (rev a2)
00:04.0 PCI bridge: nVidia Corporation MCP61 PCI bridge (rev a1)
00:05.0 Audio device: nVidia Corporation MCP61 High Definition Audio (rev a2)
00:06.0 IDE interface: nVidia Corporation MCP61 IDE (rev a2)
00:07.0 Bridge: nVidia Corporation MCP61 Ethernet (rev a2)
00:08.0 IDE interface: nVidia Corporation MCP61 SATA Controller (rev a2)
00:08.1 IDE interface: nVidia Corporation MCP61 SATA Controller (rev a2)
00:09.0 PCI bridge: nVidia Corporation MCP61 PCI Express bridge (rev a2)
00:0b.0 PCI bridge: nVidia Corporation MCP61 PCI Express bridge (rev a2)
00:0c.0 PCI bridge: nVidia Corporation MCP61 PCI Express bridge (rev a2)
00:0d.0 VGA compatible controller: nVidia Corporation GeForce 6150SE nForce 430 (rev a2)
00:18.0 Host bridge: Advanced Micro Devices [AMD] K8 [Athlon64/Opteron] HyperTransport Technology Configuration
00:18.1 Host bridge: Advanced Micro Devices [AMD] K8 [Athlon64/Opteron] Address Map
00:18.2 Host bridge: Advanced Micro Devices [AMD] K8 [Athlon64/Opteron] DRAM Controller
00:18.3 Host bridge: Advanced Micro Devices [AMD] K8 [Athlon64/Opteron] Miscellaneous Control

~$ aptitude show xserver-xorg-video-intel

Pacchetto: xserver-xorg-video-intel
Stato: installato
Installato automaticamente: no
Versione: 2:2.6.3-0ubuntu9
Priorità: opzionale
Sezione: x11
Responsabile: Ubuntu X-SWAT <email address hidden>
Dimensione pacchetto installato: 1323k
Dipende: libc6 (>= 2.4), libdrm-intel1 (>= 2.4.5), libdrm2 (>= 2.3.1),
         libpciaccess0 (>= 0.8.0+git20071002), libxext6, libxv1, libxvmc1,
         xserver-xorg-core (>= 2:1.5.99.901)

Va in conflitto: 915resolution, xserver-xorg-driver-i810,
                 xserver-xorg-video-i810 (< 2:1.9.91-1),
                 xserver-xorg-video-i810-modesetting,
                 xserver-xorg-video-intel-modesetting

Sostituisce: xserver-xorg (< 6.8.2-35), xserver-xorg-driver-i810,
             xserver-xorg-video-i810 (< 2:1.9.91-1),
             xserver-xorg-video-i810-modesetting,
             xserver-xorg-video-intel-modesetting

Fornisce: xserver-xorg-video-5
Descrizione: X.Org X server -- Intel i8xx, i9xx display driver
 This package provides the driver for the Intel i8xx and i9xx family of
 chipsets, including i810, i815, i830, i845, i855, i865, i915, i945 and i965
 series chips. 
 
 This package also provides an XvMC (XVideo Motion Compensation) driver for i810
 and i815 chipsets. 
 
 More information about X.Org can be found at: <URL:http://www.X.org>
 <URL:http://xorg.freedesktop.org>
 <URL:http://lists.freedesktop.org/mailman/listinfo/xorg> 
 
 This package is built from the X.org xf86-video-intel driver module.

~$ aptitude show xorg

Pacchetto: xorg
Stato: installato
Installato automaticamente: no
Versione: 1:7.4~5ubuntu18
Priorità: opzionale
Sezione: x11
Responsabile: Ubuntu Core Developers <email address hidden>
Dimensione pacchetto installato: 24,6k
Dipende: xserver-xorg, libgl1-mesa-glx | libgl1, libglu1-mesa, xfonts-base (>=
         1:1.0.0-1), xfonts-100dpi (>= 1:1.0.0-1), xfonts-75dpi (>= 1:1.0.0-1),
         x11-apps, x11-session-utils, x11-utils, x11-xfs-utils, x11-xkb-utils,
         x11-xserver-utils, xauth, xinit, xfonts-utils, xkb-data, xterm |
         x-terminal-emulator, x11-common, xinput
Raccomanda: xfonts-scalable (>= 1:1.0.0-1)
Consiglia: xorg-docs
Fornisce: x-window-system, x-window-system-core
Descrizione: X.Org X Window System
 This metapackage provides the components for a standalone workstation running
 the X Window System.  It provides the X libraries, an X server, a set of fonts,
 and a group of basic X clients and utilities. 
 
 Higher level metapackages, such as those for desktop environments, can depend
 on this package and simplify their dependencies. 
 
 It should be noted that a package providing x-window-manager should also be
 installed to ensure a comfortable X experience.

~$ aptitude show flashplugin-nonfree

Pacchetto: flashplugin-nonfree
Stato: installato
Installato automaticamente: no
Versione: 10.0.22.87ubuntu2
Priorità: opzionale
Sezione: multiverse/web
Responsabile: Ubuntu MOTU Developers <email address hidden>
Dimensione pacchetto installato: 41,0k
Dipende: flashplugin-installer
Fornito da: adobe-flashplugin, flashplugin-installer
Descrizione: Adobe Flash Player plugin installer (transitional package)
 This package is a transitional package that can safely be removed after you
 installed flashplugin-installer.
Homepage: http://wiki.ubuntu.com/FlashPlayer9

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