For just general purpose I'd go with
Ubuntu, which is a nice and simple install and so far has been painless to use, or
Kubuntu if you like the KDE interface better than the GNOME one. My dad has been using
SUSE, and he likes it so far, and it seems to be fairly user-friendly. So I'd start with those and see how you like them. Ubuntu has a live CD thing going on, so you don't even have to install it to test it out, just download the .iso and burn it to a CD and boot off it.
Now if you're doing things like Photoshop or Flash, then you might want to consider getting an OEM version of XP. If you're used to Photoshop's interface, it'll be harder for you to switch to GAIM, even though GAIM is reported to have the same capabilities as Photoshop(I haven't been able to test that myself since I can't get around the interface). There is one called
Ubuntu Studio, which is like XP Media Center, except Ubuntu, so that's another choice for you. Open Office.org is a really good replacement for Microsoft Office too, so any problem with switching will have to do with Adobe software. You can also get
WINE, which will let you run Windows programs on other OS's, but I haven't tested it with memory intensive programs, so it may or may not work well for you.
Your computer should be able to run any of those without any problems at all, so try out Ubuntu, or Kubuntu, or SUSE, and if you like them, great, if not, go ahead and pick up a copy of XP, or wait for Windows 7 since it seems stable so far, but it won't be out for a little while yet.