I would do something that JM eluded too. Get a new computer to do your design work on and run your SP300 off of your current computer. That way your new computer won't be tied up with ripping your print jobs. And yes you can design and rip on the same computer at the same time, but in the end, you'll be happier with the processes on seperate computers.
Either an Intel or AMD chip will suffice, I like AMD personally
Go for 2GB of RAM, its worth it.
Figure up how much hard drive space you want, and double it
Unless it'll double as a gaming rig, you don't need more than a $100 video card, tops.