Erm..
Apple's new macs uses a new dual-core processors which shares a single cache (a very fast memory on the processor itself). The 2 cores can "talk" to each other via a super-fast bus, this allows true utilization of the 2 cores on the CPU. Current Pentium D 8xx-series doesn't share the same core, and I am think they talk to each other thru the Front-side-bus (much slower compared to a delicated bus). It is still faster than single core Intels, but doesn't really gives the true benefits of dual-core processing to the end user.
Anyway, the current Intel processors used on Macs (or simply called Intel CORE) will be on the roadmap for like only half a year.
In the 2nd half of the year, Intel will launch a brand new architature (Current Intel processors based on the old NetBurst Architature from Pentium 4) which will be used in Intel processors in the next few years. That processor is in my plans for a next upgrade.
Anyway since you need a computer now, let's put that topic a side. I always feel that Intel processors are still good buy compared to AMD. (Maybe that is because I have bad drivers issue with AMD PCs.) I think Pentium Ds would be good buy.
RAM? My next computer will definately have 2GB of RAM. My 2-yr-old P4 2.4Ghz has 512MB. This, until a year ago, was more than I would ever need. (I would not upgrade to 1GB now because I do to invest into DDR rams right now. My next computer will definately be DDR2, my RAMs would be useless) Now I feel like 1GB will be more than what I ever need, but to be safe, I would take 2GB.
GeForce 6600GT is still the current best balance for price/performance. But I can't help feeling that nVidia would come out with a 7xxx-series mainstream performance card to replace 6600 (7300 was just launched for the entry level card)
Screen? DVI and 19-inch and above!