I can heartily recommend building your own PC - you get exactly what you want. You tend to get a lot of tat with the ready-made PCs which ultimately you do pay for - I'm thinking of the crappy bundled software here.
The other advantage is that you can reuse a lot of bits - floppy drives, cables, monitor, keyboard etc. I spend about £300 every couple of years and get a new PC to all intents and purposes. If you focus your spending on a decent m/board, memory and processor that's about all you need if you're happy with your existing monitor etc. I still have a GeForce 2 graphics card which peforms pretty well despite all the hype about the latest greatest cards. I think I will have to consider a new one for Half Life 2 and Doom 3 if they ever make it onto the shelves.