What video card should I buy with £100?

[Wreford-Brown]

I almost hijacked a thread by Zachstar on ATI graphics cards but now need some more in depth advice. I also need to thank Zachstar, CaptainHaplo and CCIP for the help they've already given. My system:
MS Windows XP SP3
Pentium 4 3.06 GHz
3.0 GB RAM
NVIDIA GeForce 6200 LE

It's the graphics card that's causing me headaches as it's giving me 5 to 7 FPS on SH4 and I reckon Santa may be able to bring me a new one.

I'm in the UK and reckon that shipping and customs will prevent me buying from the North American market. I've only been onto PC World and overclockers.co.uk websites so far and have found the following in budget:

PNY GeForce 8800 GT 512MB PCI-E Graphics Card
BFG GeForce 8800 GTS OC 512MB PCIe 2.0 Graphics Card
XFX 9600 GT OC 512MB PCI-E Graphics Card
PNY GeForce 9600GT 512MB PCI-E Graphics Card
PNY GeForce 9600GT XLR8 OC 512MB PCI-E Graphics Card
PNY GeForce 9500GT 512MB PCI-E Graphics Card
Powercolor ATI Radeon HD 4830 512MB GDDR3 TV-Out/Dual DVI/HDMI
Nvidia Geforce 9800GT 512MB PCI-E

The questions are:
Is a higher number a better card?
What is the difference between a regular card and an overclocked one?
Does the brand make a difference (PNY vs XFX etc.)?
Are there any better websites I can browse?
Is there a simple comparison site for graphics cards?
How do I work out what my power supply is and will it make a big difference?