Which GFX Card is better?

Hi everyone. Im new to this forum, so far so great. Anyways I have a ATi 4850 Toxic 512MB card, and it seems to be okay, does the job. I normally play Counter Strike Source and sometimes other games such as Crysis and stuff like that. My uncle got a BFG NVIDIA GeForce GTS 250 OC 1GB - I was wondering which card is actually better? The ATi 4850 toxic gfx or the Nvdia 250 ?? So maybe I can do a swap lol. Thank you.

On specs they appear very similar.

ATi 4850 toxic

GeForce GTS 250

The latter obviously has twice the on board memory. 1GB / 512MB although both cards come in multiple memory allocations.

Without personal experience of these particular cards I'd suggest reading a few reviews:

ATi 4850 toxic

GeForce GTS 250