As it turns out, the motherboard in one of my machines here is G45 based and has the X4500HD. I'd forgotten about it because at some point in the past, for a reason I can't remember, I put an nvidia card in it. I'm thinking about grabbing the latest Ubuntu live-CD and giving it a go to see what the experience is like (it's shamefully running Windows at the moment >.<).
Truth be told, as I said above, my budget is more likely to make the decision for me. The idea was to have a mobile PC as a nice addition without breaking the bank, not to spend way over the odds on something just to get an nvidia GFX card.
It really is a shame that Dell don't seem to want to release the Studio 14z over here in Europe. I'd order one tomorrow if they did, despite the RAM thing.
Anyway, thanks again to everyone who took the time to reply. I'll report back when I've had a chance to play with Ubuntu and the X4500HD.