hey turtle,
i hit this wall too when i wanted to start using sonar 4. i was using a p3 with cakewalk home studio (2002) for a long time, and then got a p4 desktop and a turion 64 laptop. honestly, it looks nicer, but for my purposes, where i seldom record more than 4-5 (or more than 2 more often) tracks at a time, but i couldn't find a whole lot of difference between the two. admittedly, i didn't start using higher sample rates (even on the desktop) until i got the laptop.
you might want to do a little more research (google searches essentially should do it) to see if there are any compatibilty issues with your interface and whatever computer you look to buy (processor and especially chipsets). i have this mackie onyx 1220 that works great with this laptop, but has all sorts of problems with my p4 desktop.
also, maybe look at craigslist too-- shipping costs for computers can be pretty unreasonable.