Allow me to do a blanket answer, due to (as Steenamaroo said) a vague question...
According to you, $600 is expensive. The CHEAPEST Macbook is $999 (Macbook Air) without any discounts from being a student or any other garbage like that, so those are apparently out of the question. Macs aren't actually any better for audio than PCs, as they both use the exact same hardware, and they both use the same software (unless you're using Logic, Garageband, or Sonar... But we'll assume based on your budget, you're using Reaper or Cubase or some other multi-platform DAW).
Moving on...
I guess I stand corrected. Macs TECHNICALLY are better for audio, assuming the PC you use has an AMD processor. AMD is designed for working with numbers in different ways than Intel is, and Intel is the preferred brand for working with audio. So if you buy a PC, stick with Intel, despite AMD being cheaper.
I'd suggest at least an 8GB RAM laptop, if you're doing this semi-professionally. 4 GB will do in a pinch, but 8GB isn't much more expensive, and it'll give you a lot more to work with.
As Steenamaroo said, you need to know if you're doing USB or Firewire.
If you want better equipment, Firewire interfaces are the higher end ones (unless someone jumps on a USB 3.0 interface anytime soon), so I'd make sure you had at least one IEEE 1394a port on there (also called Firewire 400).
Obviously, higher track counts means you need more RAM and more plug-ins means a faster CPU, etc. etc. If you're going PC, I suggest HP as the brand, as they're well known and their tech support hasn't sucked for anything I've needed so far.
Anyhoo, if you want a shorter answer, be a wee bit more precise, and I'd be willing to help out. =]