1) What are the complete system specs of your friends PC - CPU/PSU/RAM/ETC?
AMD Athlon X2 4600+ (AM2)
1GB DDR2 667 RAM
ASUS M2N-E (AM2+)
160GB HD
GeForce 8400GS PCI-E
WinXP SP3, all updates
Corsair VX450W
DVDRW
We're considering further upgrades, but we'll probably stagger them over the coming year. My feeling is to upgrade the graphics first, probably RAM with it to 4GB (with a view to go to Win7 64 in time, I'm aware of the RAM limit on 32-bit), then CPU when the Phenom II range are in danger of disappearing off the shelves (I've checked the CPU support list for that board). Of course most of this is based on how well the son feels that the game plays on the current setup.
2) You ruling out AMD cards because of any recent bad experiences you personally had or because of what you read on the internet? You'll find loads of people in these forums ready to vouch for AMD drivers, including myself (I've been using AMD cards, in single GPU configuration, for nearly 3 years without a problem), that will most likely say any difference between AMD and NVIDIA drivers is negligible (although I've no idea how these cards behave on this specific game).
Personal experience, which starts at around 1999, then a few years ago (guessing 2008) I bought an ATI graphics card, and while it still works to this day (though it is only being used for basic 2D work like Office), the XP welcome text shadow is corrupted (pink), and I built my own system in 2010 with a 5770. Poor fan management leads me to use third-party software to keep the fan speed low while still maintaining a reasonable GPU temperature (because AMD OverDrive increases the clock speeds to 75% full when I change the fan speed), occasional graphics corruptions (but very artistic I must say) in StarCraft 2, I had to downgrade LibreOffice to 3.4 when the latest release had problems with page and table borders on my system. My current (5770) system used to hang if I played one WMV after another several times, regardless of GPU acceleration (in WMP or the extra installable bit in the AMD driver), as well as a regular graphics corruption. Luckily a recent driver update fixed the WMV problems.
Comparing that to >10 years of zero issues with nvidia cards except the very occasional slight imperfection in rendering (like a shadow with a very thin line through it). The only reason that I went for AMD for my own system was apparently at the time AMD was way ahead in performance terms.
I'm aware of nvidia having driver issues over the years, AFAIK particularly when Vista was released, but I'm primarily going on my own experience and a few of my friends'.
3) At £80 (you can find them under £80 at overclockers.co.uk, depending at the rest of the system specs, a 5770/6770 (same card, different numbers)
5770/6770 - whoa, I wasn't aware of that. IMO, that's a bit deceptive of them.