If you would like to enable Quartz 2D Extreme (see this hint for more info on it), but don't have the VRAM to support it, you can change the minimum requirements by following these steps in the Terminal:
$ cd /System/Library/Frameworks/ApplicationServices.framework/Versions\
/A/Frameworks/CoreGraphics.framework/Versions/A/Resources
$ sudo pico Configuration.plist
Once in the editor, change these values to match your machine (use System Profiler to get the required details):
<dict>
<key>Quartz2DExtremeEnabled</key>
<true/>
<key>Quartz2DExtremeMinimumVRAM</key>
<integer>64</integer>
<key>Quartz2DExtremeMinimumDRAM</key>
<integer>512</integer>
<key>Quartz2DExtremeVendorDisabled</key>
<array>
</array>
</dict>
[robg adds: I haven't tested this one, and I can't imagine that the results would be very good -- Quartz 2D Extreme requires a large amount of VRAM to work, and if it can't find it, uses a form of virtual memory to simulate the VRAM on the card. At least, that's my interpretation after reading the review at Ars Technica (linked in the above-linked hint). I would imagine a reboot would be required to activate these changes, and I'd also strongly recommend making a backup (sudo cp Configuration.plist Configuration_BACKUP.plist) before you start editing this file -- just in case you want to easily revert the changes.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050503190059805