Two Plus Two Older Archives

Looking to buy a new PC. I'm a software guy, don't know that much about hardware. Currently running a 3-4 year old 1.8 GHz with 512 RAM and it's hitting the wall. I play the occasional game, run WMP with 3000+ songs, develop software using Visual Studio.NET 2005, have various internet applications running and generally use the crap out of my machine. I bought my machine in a weird window where I have a bizarre type of RAM called RAMBUS (I guess?) and it's a pain to upgrade. I figure I could use a new machine anyway.

So I'm looking to get a new PC (probably a Dell) that's reasonably fast. The new processor types (HT, DualCore, etc.) have me confused. I figure I'll throw 1 GB of RAM in it and probably upgrade to 2 later if I need it. What else should I be looking for? Are the 256 MB lower end ones on the XPS 600 systems ok?

Also, what is the fundamental difference between the 600 and the 400 series (aside from case), if the specs come out basically the same? I don't need a monitor, just looking for the machine. Hoping to get one for somewhere between $1000-1500...obviously don't want to spend money unnecessarily. Any opinions appreciated.