Eh, pretty much any decent computer with sufficient RAM will suffice. I personally would go with any AMD processor around 2.5 GHz, at least 2 GB of RAM, and make sure you get some good stuff, not anything anyone tries to sell you with your computer. Buy it separate and from some place like Newegg. Be sure to check sites for technical spec comparisons and benchmarks. Also, I'd recommend getting a SATA drive at the very least, and if you can get a 10k RPM drive instead of 7.2k, go for it. Make sure it has a decent sized buffer, too (at least 16 MB). These are fairly expensive, but you'll notice a difference with PT and dealing with large databases. For extra storage you can get any old IDE 7.2k RPM drive. As far as video cards go, nothing more than 128 MB is really necessary. 256+ is overkill for poker... really only necessary for first person shooters and stuff.
Really I emphasize RAM and hard drive specs over processor because the poker stuff really isn't processor intensive and the price/performance ratio is too prohibitive -- especially if you're going with dual core and all that crap. Sans monitor and speakers, if you part this out it should cost less than $600 (and you can bring this down to $450 if you spend time looking for good deals), including shipping and handling.
I wouldn't worry about Vista now, it's not really a concern. Like most people, you probably won't upgrade when it comes out. I can tell you that on my laptop (1.86GHz Pentium M, 2 GB [of good RAM], 5400 RPM drive, and 256 MB GeForce) I can do as many tables as I like on any poker site (6 at PR, 5 on the Cryptos, 7 at Party, 6 at Absolute) without any problem. The only one that gives me any problem is Absolute, when I have PT auto-importing and PAHUD running at the same time. This is mainly because Absolute launches a separate instance of an application for each table and their coders are shitty.
Keep in mind that the people writing the poker clients want to make them accessible to people without top of the line computers... you don't need to go overboard here.
Anyway, take this with a grain of salt or not... but I do have two degrees in computer engineering and I build all my own boxes
