First, you're gonna find a machine capable of supporting more than 2 Gigs of RAM in the the price range of a reasonabley young, nicely equipped used car. That outta the way, if you're happy with and confident of HP product, and you intend to do your shopping on the 'net, their website is the place to go. They offer a decent "Buying Tips" page
HERE.
Something else to bear in mind is the impending arrival of Windows Vista, the operating system slated to succeed Windows XP in Q1 '07. Essentially, the basic requirements for Vista include a processor of at least 1Ghz, a minimum of 512MB of RAM, a multi-channel audio solution, and a video solution having - or sharing - at least 128MB of RAM.
For general basics, I'd say you'll want an optical drive capable of writing to multi-format, multi-layer DVDs, support for USB 2.0 and FireWire, a networking solution, and direct digital monitor output (either
DVI or the newer, somewhat more capable
HDMI), and perhaps a built-in multi-format flash card reader. You should also try to see to it whatever machine you get - assuming it will be a desktop - will have expansion capabilties; at least one open bay and at least a couple open PCI (or better yet, PCI Express) slots. If you go with a laptop, think forward, and get more machine by a good margin than you think you need right now, as the upgrade path for laptops is limited.
Pre-purchase live chat and sometimes even phone help will be available at the website of just about any major computer vendor ...use it. And while you're on the vendor's website, take a look at their closeouts/returns/refurbished units - often catalogued in something titled along the lines of "Outlet Store" - if you know what you want, and why. While such offerings rarely are "Latest/Greatest", they're generally current or very recent production, with full as-new factory warranty, and there are bargains to be had if you look for them.