Am i going to see any performance gain from 512mb?
I play GTR2, Trackmania, maybe Oblivion again and I will be doing some 3d rendering in the near future but im not sure if the gfx card has much to do with that.
Bearing in mind the card I buy will be used for the next year till I upgrade to a DX10 card.
IMO, the extra memory is really just for "future proofing" as most games don't use that much memory yet, mind you I am currently playing Quake 4 again at Ultra settings and it says when starting the game that it requires 500Mb.
Google 256 Vs 512 and there are a few reviews and tests regarding this very issue. The 512 only wins out at higher resolutions (1280x1024 min) and higher AA and AF settings (4x and 16x).
If you are only getting one card I would recommend the X1950, it will easily beat the 7900GS on it's own unless you OC it alot then it evens out.
My cards look to be the same as the ones you are considering (mine are a Chinese card, Unika and yes they are ugly), if they are they are the same they are built on a 7800GT board with an upgraded power supply circuit, mine OC well to 585/760 and that ugly cooler actually works very well and is not noisy at all.
I only got the 7900GS because I wanted dual cards and the crossfire solution was way too expensive for me but I am very happy with it's performance.