I vote for windows 7 64 bit.
1) Using a 32 bit OS, you will be able to see only about 3.5gb, not the full 4gb you can see with a 64 bit os.
The 32 bit os needs to reserve some space for hardware areas that the 64 bit version does not.
2) Windows 7 is more secure than the 32 bit version.
3) Windows 7 64 bit will run apps with out any degradation. Emulation is not really the right word. Only old 16 bit dos programs like the original civ 2.42 will not run
4) Ram is cheap, a 8gb upgrade does not cost much. and can help with some aspects of gaming:http://blog.corsair.com/?p=65
5) If you are already running XP or vista, buy the upgrade package for windows 7 instead of oem. The price will be similar.
With upgrade, you get both 32 and 64 bit dvd's. Upgrade is also considered as retail, giving you ms support and transferability to a different motherboard.