If you are already using a GL2, the XH A1 seems like a seamless change. The XH feels very similar to the GL2 in several ways. I used my laptop to edit, it has a 1GB video card, It took 15 min to render the 30 second spot.
How beefy your computer depends on how much time you want to spend rendering and encoding. I don't think a beefier computer with a super expensive video card is the answer, patience works for me, but if you are paying someone by the hour so edit video, then the extra cost is worth it. I use Adobe Premiere Pro CS5 for editing. CS5 is HD friendly.