I would hazard a guess that you are running an Intel Pentium 4/D, and that the graphics card called Xpress 200.
This card is actually an on-board Radeon X300, which has full support for Shader 1.1, 1.4 and 2.0. It's not too shabby performance wise either.
If this is the case then you will have a PCI-E x16 slot, and if it's the Xpress 230 then you will also have an AGP 8x right next to it.
For £70, I would recommend purchasing this:
http://www.saverstore.com/productinfo/Product.aspx?catalog_name=Savastore&product_id=20013712&rstrat=12868
Radeon X1600 Pro PCI-E 16x 256MB £68
If you extended your budget then I would HIGHLY recommend either:
http://www.saverstore.com/productinfo/Product.aspx?catalog_name=Savastore&product_id=20009687&rstrat=12868
Radeon X1950 XTX PCI-E 16x HD-Ready (HDMI + HD-Component) Vivo 512MB £120
Personally I'd recommend waiting a month or so until the Radeon HD-Series is fully lunched. As the 2300 and 2600 Series are all HD-Ready, plus support DirectX10.
Still waiting for places to provide GeForce 8300/8600. Honestly it's a pain waiting for these budget cards.
An added bonus of that X1950 XTX is you'll be safe in the knowledge that anything you or your friends can play looking gorgeous on the 360 at 720p will also be true for this card. As it's practically identical, so you can have basically identical graphics settings to those on the console and expect the same performance.
It has no issues playing ANYTHING currently available, at whatever settings you choose. This card is the same price mark as the GeForce 8600, so much better value for money imo