Let me crawl out of the fetal position long enough to say that Nevins is correct on every point.
Yes you can use the unused cores in a standard cable, and you can even buy splitter boxes that will break it out for you into two RJ45 connections over one RJ45 cable.
Nevins is also right that the purpose of the twists is to minimise EMI, both radiated from the cable and induced into it by nearby noise sources. So, with thick enough cores and a high enough signal level you could get by without the twists - until the crosstalk between the pairs (the management of which is also a function of the twists) turns your data to mush.
It's amazing what you can get away with; I've done some of these things myself and I'd certainly try them again if my back was against the wall. There is a difference, though between what you can do and what you should do. If it's just for your own xbox then go for it, who cares! But I'd never put this sort of stuff into a commercial environment.
Let's face it; the standards are conservative guidelines - they are saying "if you do it this way then we guarantee you that it will work, every time". In practice it will work if you push your luck quite a bit further than that. For example, I've had 100Mb full duplex links running over CAT5 cable at over 135 metres with no problems whatever. But when you're spending ££££s of someone else's money on cabling and kit you're going to make sure you don't end up with egg on face (and a P45).
It's also worth quoting Bishop's Law here; which states that the bodge you put in as a temporary fix will then remain in active service for twenty years because it's working now and they want you to move on something else. I've twisted wires together myself and it works - but it always comes untwisted again later on, usually when you could really do without it.
Right, back to the fetal position. Now, where did I put my thumb...