For around £100 you want to get a 512Mb 8800GT. There are deals available to get one for £100, though most are priced a little higher. Don't get a 256Mb card, it doesn't perform as well as you might hope. If you have trouble getting one in your price bracket, then the 9600GT is almost as fast (the same speed in some games) and can be had for less than £100.
These two cards represent the best value purchases at the moment
And don't worry about the 4x PCIe, they are all compatible, but in theory the 4x will be slightly slower than a 16x slot. In practise I'm not sure if you can tell the difference... though I'm sure someone is bound to disagree