AE can work on comps up to 30,000 by 30,000, if I recall. So HD is no sweat for the app, as is 2K or 4K film.
You need enough RAM to preview the number of frames you want at the comp window res you want. Do a test animation at 1280×720, 1920×1080, keep Task Manager open to see RAM usage, and see how many frames you can preview. 2GB isn’t much … you should have at least 4 GB.
Disk speed isn’t a big issue with AE — that’s an NLE problem.
As for file size, render a :30 spot in the two resolutions above in the Animation codec, Photo-JPEG codec, TGA sequence, PNG sequence and see how much space they take up.
You might be in trouble if you have to render to DVCPROHD or Apple ProRes codecs, since you have a PC, and those codecs are mac-only, if I recall. You might want to consider switching over to OSX.
AE doesn’t like importing any MPEG, including HDV (unless they’ve changed something). You should convert to another format before importing those.
Anybody else?