I'd say that you'll be best off not worrying too much about the chip speed - there won't be too much of a noticable difference between say a P4 670 with HT technology 3.80GHz, 800MHz fsb, 2MB cache and say an Athlon XP 3000+ or whatever. Spend the extra on a good graphics card and more RAM for a smoother better performance.
Something like a 256Mb nVidia GeForce 7800GTX would be more than adequate, try and get at least 128Mb for heavy duty graphics stuff, especially if you have a large display. 1Gb of RAM is probably the amount to go for.
Rob