my non music job at the current moment has me in the IT field. Let me lend my 2 cents here, if I may. All of the feedback that you have received has been dead-on-balls-accurate, and some excellent food for thought. Let me add in some desert.
In the 500-700 dollar range, you are going to find a lot of very capable Windows 7 based PC's in any big box store. Warranty is nice to have because the computer companies have been known to build a lemon or two. Standard MFW is 12 months. DIY is fun (and sometimes not), and you have something you accomplished to be proud of, but the hours and hours of researching parts, compatibility, etc is consuming. Look at your personal time as you would if you were getting paid for it. How much is this machine worth to build, in personal time?
MAC vs. PC. If I could justify the money, I would abandon the PC world altogether, and go Mac. Better QA in build, simpler O/S, and tough as nails. The flip side to that is software compatibility, in any area. You are extremely limited in this area, but it is getting slowly more hopeful, as more people make the leap to Mac. Software for what we do is WIDELY available for PC, shareware, freeware, and copywritten for sale.
You are very right about USB. Almost infinite possibility for expansion with USB compatible devices. I have done many many hours of editting and matrix mixing on my laptop, tethered to all of my external drives, and equipment. Mac still likes to use firewire, but they do have USB on all machines (the MacBook Air does not have FW, just USB), and all you need is one to hook up a USB expansion bus. the speed difference between USB 2.0 and firewire is negligable, IMHO. USB information transfer across a bus between devices is going to slow you down a tad, anyway.
It sounds as if you have enough reliable back-up as far as other computers available, so the choice is not limited by this. I can tell you that almost anyhting in Best Buys add in that price range, other than the eMachine and Compaq DT's are going to be infinately fater than what you currently have now. I would not settle for anything less than dual core processors. Quad core if it fits the budget, but not necessary just for music editting. As for Windows 7:
Microsoft made a mistake with Vista. They acknowledge that to some degree, for various reasons that I will not go into now. I liked Vista. The only compatibility issue that I ever had (and eventually corrected) was with WinTaper, of all things. With that said, it took me a 7 beat intro to BIODTL to switch over once I used Win7 a bit. Less resource dependent (hmm, like Leopard/Snow Leopard), more visually appealing (again, like Mac), XP compatibility mode in professional and ultimate packages (see Bootcamp / Parallels) and the very important compatibility with your older peripherals.
Newegg carried Win XP for developers for sale at a cheap price. Microsoft does not sell it anymore, and does not support it. It was around almost 10 years. Vista is even difficult to find in Best Buy, as well, because the Microsoft empire is really pushing the herd towards 7. Now that I said that, a little more opinion: Mac allows independence and individuality. Windows really is big brother corralling the herd together. Microsoft has more customers then the Catholic church has Christians. think about it
I hope in some small way, I didn't rant, and actually provided you with at least one line of useful info