Forget about the 667mhz 2x512mb you have, for better memory performance you can get:
4GB 2x2GB DDR2 at 800mhz speed dual channel kit, from any good branded memory manufacturer such as G.Skill, Corsair, Kigstone, Crucial, etc..
try to get the ones with the lower timings options you find, and you should be fine with a good 64bits OS.
thanks for the idea, iam running a 32 bit OS, my desktop is a kind of old by the way its only a core 2 duo 2.33GHz hmm not much for a new one, its only for normal use. little bit of gaming and school works, my system is not much for hardcore use, iam around 1-2 gig of upgrades
(looking forward for 1gig coz i plan for buying a new desktop computer), if i go for the 4GB will it be a bottleneck for my system. the brand of my desktop is LG XPION. thanks juanjostorreshernndez