1. Just so we are on the same page here SD=Standard Def video and HD=High Def video.
2. Internal storage capacity is a non-issue because you'll be using an external storage solution if you are working w/uncompressed SD or HD video.
3. That is a nice PC you spec'd out but I would be extremely surprised if it ran Avid Xpress Pro (check the Pro's "specifications" page on Avid's site). You also neglected to add the cost of the Mojo ($1699 + another $75 for component video i/o). W/o the Mojo Avid Xpress Pro is DV only. For less than a grand I can get quality SD capture hardware and for less than 2 grand I can get a quality HD capture card. BTW, the S-Video i/o on your video card is not used for editing on anything other than consumer applications.
4. Stablity. From personal and professional experience I've noticed that Windows based NLEs (non-linear editors) need more TLC and have more quirks than their Mac based counterparts. This is not to say that Win-based machines are the crash-happy crap-boxes that some people on this sight play them out to be, but they do require more looking after than their Mac counterparts. NLE's stress the whole computer, not just the proc or the video card. And that's where one of the "WinTel" world's biggest strengths becomes one of it's biggest weaknesses. There is such variety of mother boards, and procs, and video cards and sound cards, and drivers, and chipsets that getting that them all to "play nice" is impossible. But having tight, flawless system (hardware and software) integration is key for a stable NLE. And the more powerful the NLE the tighter the integration must be. Just look at Avid's site. 10 PC towers qualify to use the Pro software but only 2 qualify to use the Composer Adrenaline system (and only 1 qualifies to use the non-Adrenaline Composer). But one dual gig G5 is pretty much like every other dual gig G5. If MS made a closed hardware system and a specific version of XP to run on it I'm sure it would be just as smooth and stable as the best Mac. Conversely if Apple released an x86 version of OS X I don't think it would be any more stable than XP is. Like I said before, 5 or 6 year old PowerMac 9600 based Avids aren't still in regular service because they are the fastest thing on the block.
What I have on a G5+FCP that you do not have on an Avid Xpress Pro (off the top of my head).
1. FCP + 3rd party SD capture card has superior uncompressed SD abilities. The Pro + Mojo is limited to 1 stream of uncompressed SD video (and seems really geared for compressed/off-line SD video). The Mojo also lacks SDI i/o.
2. Superior Audio i/o. The Mojo, AFAIK, lacks balanced audio i/o
3. Native 8-bit and 10-bit video in FCP. 8-bit only, if that, in XPro.
4. Native HD editing in FCP. You'll need an entirely new Avid setup to get HD if you own an XPro. All I need is a HD capture card.
5. Avid Xpress Pro, because it is Avid's "budget" system lacks editing/interface features that FCP has (the FCP feature set is more comparible to an Avid Media or Film Composer).
6. AFAIK XPro does not support any other capture hardware other than the Mojo.
7. FCP 4 also comes with Soundtrack, Live Type, Cinema Tools, and Compressor.
The Pro is basically the XpressDV 3.5 upgrade w/limited uncompressed abilities when the Mojo is attached. It's not a bad system but it is, for the most part, limited to DV. If you are going to use Avid as your example you should compare a G5+FCP to a Media or Film Composer. Of course, you could put together an entire FCP based edit suite (decks, broadcast monitors, video scopes, etc.,) for less than the cost of a single Media/Film Composer system by itself. That's one of the amazing things about FCP. Its low price means that it is easily attainable for the no-budgets, but it's so flexible and powerful that it is being used more and more by the big budgets as well.
/rant.
BTW, any reason you didn't "buy" the RAM and HDDs for the G5 you spec'd at Newegg (or Crucial for the RAM) too instead of quoteing the prices from the Apple Store (which are known for being over-priced)?
Lethal