But if he wants Crossfire....anyway I've seen people with Nforce 4 problems saying "never buy again" so all have bugs. My board has been rock stable, it's got the Mosfet whine though, and I can't test Crossifire yet.
Personally I would just buy a single X1900XT, wait until framerate drops until annoying level, decide if a X1900 master card or just replacing the single card again. You have a choice, if you get a Nforce basically single ATI, single Nvidia or SLI. You're going to pay more for a SLI or Crossfire but what's a extra £20 for a mobo? At least you can get dual cards, if you buy a single GFX slot mobo and want dual GFX later means new motherboard, and probably buy that time CPU for that socket are old hat, new memory pin type etc. Basically new PC. Personally I would opt for a dual core CPU also.
As for Nforce 4 mobo- Asus A8N-SLi Premium nForce4 SLi (Socket 939) PCI-Express Motherboard (MB-111-AS)