You could build it yourself...
If that's not your cup of tea I'd recommend Dell or Gateway.
I tend to lean a little more toward Dell than Gateway, both are quite good though.... check out their websites and see what's available. See which one offers the best deal for what you configure and there ya go...
Have a client who just got a Dell... for around $850... free shipping and doubled the RAM for free. Nice machine, P4 2gig, 512 on the RAM.... only a 17in. monitor... Nvidia 4200 etc....