The short answer is: You're spending close to $1,000 on what I assume is a gaming machine, including a $300 video card ... don't mess it up by throwing in one old Frankenstein component to save $50.
I don't even think the boot drive is necessary in your case - certainly not to the point of going to all that trouble to jury-rig it. If I were in your shoes, I'd just order it as-is, and use the 1TB Barracuda for everything until you can afford the SSD. No, it's not the "ideal gaming drive," but it will do the job just fine in the short term, and who knows, it might turn out you don't even care.
If you can live with 300GB of space, another option would be to put up an extra $25 and get a 10,000 RPM Raptor drive, which may not be as fast as a SSD, but is still SUPER fast ... I can tell you from experience that it serves the gaming/general use dual purpose extremely well. It's whether or not you think you'll need more space that's the issue. I had such a system for years and was very happy with it, but it may not be for you.
Course ... for not much more, you are then getting into the territory of 240GB and 256GB SSDs, which is not a hell of a lot different space-wise. At this point, it's really up to you. But any of these three options I'd rather have over taking an old (likely 5400 RPM) external drive and playing it out of position.