Probably best to start your own thread (though a mod can split these posts off if they want to). Why do you need an nVidia card specifically? There's nothing wrong with them, but you could be cutting your choices down if you mandate that.
Performance wise you'll be more than happy with anything from the 4830 upwards by ATI, or the 8800GT upwards by nVidia. Best deal I could find was this 9800GT at £92 :
http://www.scan.co.uk/Products/512MB-Palit-9800-GT-PCI-E-20-(x16)-1800MHz-GDDR3-GPU-600MHz-112-Cores-2xDL-DVI-I-HDTV
Though you could get away with a slower card certainly (what resolution are you playing at?). In some ways your needs are best served by the last generation of mid-high ranged cards, which are hard to find new - I would honestly be tempted to grab a bargain 8800GT from ebay if I were you.