Does anyone here have any insight into graphics card hardware for Doom3?

I have a budget of about 100-120GBP for a new graphics card, my sole intention is to get a better framerate and display quality over my current GeForce MX420 64MB card.

In various places where I've been reading, mention has been made of Doom3's potential to exploit video memory. As a result, I don't know whether to go for a slower card with more video memory or a faster card with less.

The cards I've been looking at are based upon the nVidia 5700LE (256MB) and nVidia 5900XT (128MB) chipsets.

Can anyone offer any insight into the potential display quality versus framerate tradeoffs between these two cards? Also, perhaps I'm barking completely up the wrong tree here; considering my budget, what other card(s) should I be looking at? Faint memories of ATI owners and their card's driver problems are steering me away from that brand but perhaps I should consider those too?

[I know that perhaps there are probably a dozen other places where I should take this post but I respect the people and their knowledge on this board more than other places.]