The GT6600 should work quite nicely as should the sb live. As for mobo+cpu combinations for older stuff something like an athlonxp + nf2 based mobo should be a decent choice (damocles some time ago made and sold such systems, so support there can be pretty decent. For more recent stuff I personally swear by intel socket775 based mobos. In particular anything with an p45 based core logic and ich7/8/9/10 i/o controllers. Throw in a cheap core2duo/quad and in amiga terms youve got yourself a nice powerful little setup. Ive also had decent luck with g31/33/41 based mobos as far as socket775 in concerned. Recent(ish) AMD wise you cant go too far wrong with an ARESOne system (although I suspect youre more interested in building from parts?). One thing worth mentioning though is that there's a bigger difference between an athlon2 and a phenom2 as there is on more mainstream Oses, due to the lack of shared cache (on multicore systems with a shared cache architecture you'll end up one active core with a very large cache).
Feel free to ask anymore questions if you have them. Ive fooled around with aros on a lot of hardware over the last decade or so, so have picked up a pretty decent idea of what works, and to what degree.
Unfortunately it can be a bit of a lucky dip sometimes, even when armed with as much info as you can get ahold of. There's enough subtle differences with hardware that sometimes even mirroring someone elses fully functioning system can result in a system with quirks. Having said that though its not nearly as bad as some people will suggest.
