There are only 2 graphic chip makers: ATI and Nvidia. The latest video card from ATI is the X800 and Nvidia got 6800. The last time I checked, the X800 was selling for about 800 Canadian, so divide that by 2.5 and you should have pound sterlin. However, video card a generation back should be cheaper and should be able to handle the job (something like an ATI Radeon 9800 or a Nvidia GeForce 5).
As for CPU, AMD and Intel are both good but I find AMD more economical. Any CPU around 2.8 GHZ should be good but avoid cut down verison like Intel Celeron or AMD Duron if you can. They both got less L2 memory. I recommand at least 1 gig in RAM since Windows XP use a lot of memory to run. Also, get well known brand of RAM (like Kingston) if you can afford it. Sometime generic stuff aren't the best.
What flight sim you plan on playing? I got a AMD 3000, 1 gig of RAM, but an old GeForce 4. I only get like 30 FPS while playing Lock-On Modern Aircombat and I got the graphics set to low or med. SimHQ got forums on just about every flight sims out there.
www.simhq.com