My pc has only 512mb ram so its a bit slow. i have been offered 2gb (2x1gb sticks) cheap. will they work ok with my pc.

My pc is a E machines with a pentium D 2.80 ghz processer. Windows 7 32-bit The current memory is a samsung 512 mb DDR2 pc2-4200. What i ahve been offerend is Corsair DDR2 PC6400 800MHZ. Will it work fine or will it cause issues?

The reason i ask is that before xmas i bought a 1gb DDR2 stick and it caused the computer to crash alot. when i removed it and went back the 512mb stick it worked fine again. Could this have just been a dud memory stick?