lol don't quit, just peek around on here use the search function and you'll find all you need to know. In order to simplify things 8800 GT currently best VFM card and an excellent performer in it's own right. below this budget ( there is nothing above the 8800 GT which can match its power per pound ratio ) then consider a pre owned 8800GTS either the 320mb or the 640mb.
You haven't mentioned the rest of the spec of the host machine or a budget? but I'll break it down for you
Budget £150-750 8800 GT no question
Budget £100-130 8800 GTS 640MB ( 320 MB at very lowest end of that scale circa £100 )
Budget < £100 1950 Pro
of course if you're teaming these up with a celeron 2.2 and 512mb of ram with a 300W no name PSU then forget it, sell the old system and start from scratch.
"with 1024mb as opposed to 512mb. Surely that makes a difference?" not always and not on lower spec cards, the gpu will run out of steam long before the amount or ram it has becomes an issue, google reviews of earlier cards such as a 256mb 6800 series versus the limited 512mb editions that came out or try the 256mb V's 512 on X800 from ATI. I'm running a 7800GT which differs very little in real world performance from a 512MB 7800 GT/GTX.