It's all relative. I have a low-end machine right now, with 1.2 GHz and 512 MB and Radeon 9200. It can barely run Half-Life 2, but it chokes on FarCry. I wouldn't even consider running Doom3 on it (not that I would want to anyway).
Most likely, I'll be upgrading to a Pentium 4 with 3.0 GHz with 1 GB and ATI Radeon 9600 (128 MB) this summer. And hard drive space? It depends on what you do with your computer. I'll probably get 200 GB Seagate, since it only costs around US$70 nowadays.
It'll probably cost around $1,500 to put it all together, which is reasonable.