I just installed Windows 7 Ultimate 3bit on my family pc which is a C2D E4500 @ Stock + 2GB DDR2 800 RAM and the performance is so so much better. The performance is total bliss now, absolutely perfect. Being new it runs perhaps slightly better than my PC too in some respects perhaps, tho mine is a Core i7 @ 3.675 + 4gb ddr3 1600 cas 7 RAM!
Install 32bit if you have 2GB or less. If you have 3GB or more, go for 64bit. And make sure htat you do have at least 2GB RAM.