To be honest, any of AMD's (or Intel's) processors could run what you want it to, with no problem at all. My outgoing Pentium III 500 happily ran IE, Firefox, Winamp, Word and Outlook at the same time with no discernible lag (although it had 640MB).
With AMD's new M2 Processor sockets arriving soon, it may be a better investment to get the cheapest cpu/motherboard/memory deal you can, but a better video card such as a 7800GT (as has been said, it's rarely the CPU that limits frame rate). I can't imagine that you would see any difference running those apps on a 4800 rather than a 2800. I *would* go for a 300GB drive rather than 250GB if you can (shop around, the difference can be <£10), preferably one with a 16MB buffer.
Personally, I went with the X2 3800 and don't regret it for a second. I use some pretty heavy duty graphics and engineering software which really benefits, and dual core upgrade patches are already available for some games (Quake 4, Call of Duty 2 etc).
Mike