which one is better??

GIGABYTE 8600GT 256MB GDDR3, Silent-Pipe II, HDTV, 2xDVI, PCI-E

or

Sapphire HD 3650 512MB GDDR2 DX10.1 HDTV DVI PCI-E‎

for games like crysis, Mercenaries 2

Both would be underpowered for Crysis, you'd be playing a low-medium settings and it would also depend on your monitor resolution. Here's a handy graphic card hierarchy chart that shows relative power of different GPUs.
http://www.tomshardware.com/review [...] 011-6.html
Both those cards are on the same tier, so no real difference. Let us know your budget, where you're shopping at (newegg if USA usually) system specs and resolution and someone will come up with some ideas.

This card on sale at Newegg would perform about the same as the 2 you had listed (same tier), but is only $20 after rebate.
http://www.newegg.com/Product/Prod [...] 6814121082
but if you want good gaming performance with modern games, you'll have to upgrade.