For the programs you wish to use (mainly Creative Suite 2 products) I'd stick with Windows XP. Adobe has no intentions of updating CS2 for full compatibility with VIsta, and, speaking with a rather truthful representative in recent months, their 'solution' is, "Buy CS3."
Being a Vista beta-tester, I can say there are redeeming features of Vista, but there are many pitfalls and blunders. In short, I don't think there's enough 'wow' in there to convince loyal XP users to migrate now.
Dell does indeed still sell systems with Windows XP, but not from their mainstream desktop and laptop lines. The OptiPlex, Precision, and Vostro systems of desktops and the Latitude, Precision, and Vostro laptops all have XP Home/Pro options.
I support the idea to build your own system, but only a scant few component manufacturers give any decent level of tech support and warranty support (not that I've ever needed to exercise a warranty myself, but I've heard of other people's misadventures with getting parts replaced or serviced). Assume that, should you build your own computer, you are on your own when it comes to tech support. If warranty support is something you want or feel you need, go with a Dell or HP.
"Once upon a time, the unattainable standard of beauty had to be met by at least one real person. Now in these days of Photoshop, that model doesn't have to even exist." -Vashti Bandy