After finally spending some serious time with the 360, I can tell anyone who missed out at launch this:
Don't worry about it. Seriously. I would equate the jump to be about equal to Dreamcast to XBOX: Good, but nothing jaw dropping. (I still consider the gap from DC to PS2 to be a step down)
Sure, there are noticeable differences, but all in all, if you have an XBOX, you aren't missing out on anything... Go buy Half-Life 2 and you'll be just as happy. And when those second gen 360 games start popping up in the spring (Gears of War), you'll be able to walk right in and buy a pro package, probably for $50 less than they go for now.