snuffleupagus, you have a powerful computer system. you just need the hardware and software to make it work nicely.
there are 2 things you will need to consider here.
1. choice of hardware
this will depends on the number of inputs and the kind of inputs you need. since you are doing recording mainly for yourself and not a live band, would you be contended with being able to record 1 instrument at 1 time?
the cost of these audio interface depends largely on the number of inputs it has and the quality of it.
at the higher side of your budget, you can consider a Digidesign Digi 002 Rack
http://www.sweetwater.com/store/detail/Digi002R/
this come complete with the hardware and software. just plug in and you are ready to go.
on the lower end of your budget, you can consider something like the Line 6 TonePort UX2
http://www.sweetwater.com/store/detail/TonePortUX2/
this is so affordable that you can just buy 1 and try to see if you like it.
Since you are using a digital drum, you can actually record using MIDI. Quntize the notes and adjust all the dynamics using MIDI and then record it to audio for mixing.
2. Software
Most software has some kind of demo that you can download and try out. And the good news is, most hardware you buy comes bundled with some software that you can use.