I may be wrong, but the only thing you should care about is the amount of ram relative to the processor speed. My personal recommendation would be to get:
a minimum of 6 gigabytes of ram, 8 being ideal, 10-12 being optimum-overkill.
a minimum of 1.9 GHz. 2.1 being ideal, 2.5+ being optimum-overkill.
hard drive space, to your needs.
operating system, to your needs.
Screen size, to your needs.
++ if you can get a 64bit computer.
You will have to compromise in some areas to stay within your budget. For example you might get a smaller hard drive for a bigger screen, etc.
Don't worry so much about the brand (dell, hp, etc...) as the stats. If your tablet has a usb connection and works with your current laptop, I can't see it not working with new computer (there's always backwards compatibility). Should also work just fine with newer versions of Photoshop.
I'm told Apple is a better choice when it comes to artists, but that's as far as I can help you. Now if you were a gamer or a video editor...
The computer I built 2 years ago:
3.4 GHz 64 bit 4 cores
12 GB of ram
600 GB HDD
Windows 7.
It's still above average. Which reminds me, I need a new graphics card