buy more RAM.

if you are still on a DDR2 platform, then it is cheaper now to buy a DDR3 motherboard and 16 gigs of DDR3 than it is to buy 16 gigs of DDR2.

SSDs have come down a lot in price, and are great when they are working, the problem is that there are still some very serious issues with their reliability/stability.  either way they do not play a major role in poser performance since with enough RAM you will be seeing very very little HDD activity - certainly not enough to justify an SSD just for poser. spend the money on RAM/processor upgrades instead.

IMHO the Phenom II x6 is the sweet spot right now for price:rendering performance.  their price has come down a lot, they overclock to 4ghz easily on air, theyre faster than the '8 core' bulldozer and are up there with the higher end (and much more expensive) i7s in raw rendering performance (but not in gaming and multimedia encoding).  just make sure you either get a board with 8 phase power or, if you do get a 4 phase power board make sure its not an MSI and you heatsink the mosfets - or buy an ASUS board - their BIOS will throttle your processor if the mosfets are about to fry.