Ok, so it does do PPPoE native (not sure what hit this means for the XBox in processor speed - it was a noticeable hit on my father's old PIII450) but that means unplugging the rest of your computers from the 'net to go Live if your router isn't supported. I do use a SMC, think its a 7004AWBR (bought back when these still went for $400+) never did anything but plug it into the router and off it went. I don't actually use the Live service as often as I should considering the cost - can't say when the last time was that I gave it a try. Hopefully it still works - I did configure my router to shut out unknown MAC addresses on top of the WEP for security and might have inadvertently shut out the Xbox.
Your best bet is to try the usual suspects and keep an eye out for deals. I've seen wireless routers on sale a number of times at FS for $40, but after mail in rebates etc. If your router is fine other than XBox Live then there's probably someone you know who can use it or sell it for a couple of bucks to help cover the costs. NCIX can have some decent deals too but shipping eats into that a bit.
Jethro - LG 60PG60, NB 3.0, Yam HTR6190, Polk, Harmony 680, Xbox360, Xbox One