What I would recommend is getting the X-treme R500 from MESH and customizing it slightly.
Click on "tell me more" under the X-treme R500 and then click on "customize". Then go down to memory and change it to 2048MB DDR400 Memory - PC3200 (2x1024)[upg £ 75.00], then go to the video card and change it to 256MB nVIDIA GeForce 7800 GT - TV-Out + DVI - PCI-Express[upg £ 120.00].
Its a shame you can't build your PC because you would save a few hundred quid.
Intel Core 2 Duo E6400 @2.4Ghz
Connect 3d X1800XT 256mb
G.Skill 2GB DDR2 667Mhz
Belinea 19\" Widescreen LCD
Antec Super Lanboy
Seasonic ATX v2 430W