I would say get the 2500k and spinpoint hdd for 308 and get a cheap say 32GB SSD for the OS for around 80 bucks and you're still under 400. Unless you get a substantially large SSD you'll only benefit from boot times with the OS and whatever apps you can squeeze on it. When z68 comes out and you can use SSD caching, overall performance should be insane... The only reason I'm not waiting for z68 is because there is no telling how much the mobos will cost, and I don't have money for a SSD anytime soon lol.
These will handle the OS and a little extra
and if you feel like dealing with a $20 MIR
another thing to consider, how much of your 500GB hd is being used? if it's not a lot then sure, get a larger ssd and just use it, but if you're running out of space get a 1TB drive.
Or buy a couple more 500GB hds and set up a RAID to increase performance and reliability. 2 more 500GB disks and you can set up RAID 5; 3 more you can set up RAID 10