I'm a bit curious why you don't think you can afford a Mac. If you want a really good PC for editing, you're obviously talking about a high-performance machine, which is going to cost a pretty penny whether you go the PC or Mac route (and sure, Macs may cost more than, say Dells, but I would certainly be willing to sacrifice a little performance to gain a lot of usability). The only way you're going to match the performance of a G5 is to actually GET a G5 (it's not just the processor; it's all the hardware AND the OS that contribute to the performance).

If you've already invested in software, though, I can see why you would be reluctant to switch to a Mac (although the new Intel Macs can now run Windows). I'd say that HP makes pretty solid computers, as does Sony (although they tend to be more expensive). I'd stay away from Dell as I don't know of a Dell owner who hasn't had at least moderately serious problems with their machine (myself included).

As far as processor, the best thing would be to get a dual-processor or dual-core machine, and/or a 64-bit system. Failing this, you're going to sacrifice some speed, but certainly any processor beyond 2GHz makes for a perfectly capable editing system.

RAM is important. Get at least a gig. The biggest thing, though, is to get as much hard drive space as you can afford. Bare minimum of 300 Gigs (depending on what kind of projects you work on, and how many you work on at once).

And if you can, invest in a video card that supports two monitors. It is indescribably more easy to edit if you have a second monitor.

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Andrew Gingerich
Exploding Goldfish Films
Check out my vodcast on iTunes: http://phobos.apple.com/WebObjects/MZStore.woa/wa/viewPodcast?id=96931870