Graphics Upgrade... Suggestions?

Hello. I have a 2006 mac pro (macpro1,1). I think its time that i update the graphics card because i started gaming again and the 1900xt just wont do. I'm not rly a gpu guru so i got some questions. Firstly, is the memory such a big factor? I looked at the ati hd 3870 which looked good at the benchmarks, but its only 512mb, just like older cards. Is this a downside? Or more memory is just useless unless you do stuff like photo and video editing etc?
So, lets say i get this card, is there any way to connect 2 of them? Like crossfire or something? Any better cards that would work on the macpro1,1?