I'm totally torn as to what I should do here :/
at the moment I have my gaming pc
i5 755 CPU
seasonic 500w psu
radeon 5850
motherboard: p55-ud3r
I'm torn, do I turn this into a mining rig or build a new one?
I have $400 to play with... can boot off USB
do I buy another 5850 for this machine? buy 2 5850s for a new machine? or secret option C, your suggestion?
The 5850 is a good card for mining so I'd recommend going for another 5850 and the Seasonic 500W (Depending on model) should be good enough for two of those... not recommended for long term but I've done 2x 5870 in Crossfire on those before for gaming.
Depending on your purpose for mining, you might want to see how the price of bitcoin stabilizes in the next few weeks before pumping more money in. If it dips sharply and you're not able to simply hold on to your bitcoins for the long term, it may not make sense to invest more.
2x5850 would also be fine for gaming in crossfire mode but tri-fire don't give you that much more and you'll definitely need a new PSU to handle that.
so I've decided to get a second 5850 (when I find one in Australia) and a 900w PSU
if the price of bitcoins looks good, I'll chuck in either a third 5850 or a 5830/5870 depending on how I'm going, thanks for the help