God, don't even consider buying HP/Dell/etc/etc if you even THINK you might upgrade something in the future. You'll just get fucked sideways in the end.
Also, Mac + games = no way, Jose. Vista is pretty sucktastic, too. The only reason you will upgrade to Vista is for DirectX 10 games, none of which are even released. Stick with XP for now (read: until Service Pack 1 comes out) and you'll be fine.
As for hardware, I'm curious about the details of your friend's Dell. For my own curiosity (and maybe others'), I'd like to see if we could price out a system that has better specs for the same or lower price. Not to mention, you'd get parts that have a better build quality and warranty.

It's a well-known fact those companies use shit parts for their computers.
I ran over to NewEgg and priced out something there for ~around~ the same price as your friend's XPS.
What I selected here is some really high-end stuff. All name brand and reliable, and I'd be willing to bet that it'll blow that Dell out of the water. Keep in mind that you could select other parts in each category that are cheaper, like a case if you don't like the one I chose. Or a smaller hard drive if you don't need that much storage. Also, you can overclock this sucker to the nuts if you're into that kind of thing. :P
Jam it back in, in the dark.