an even cheaper option could be a phenom x6 for 100 bucks and maybe you can upgrade your memory to 1866 or 1600 if your mobo supports it, vengeance in both forms for 2x4gb is like 50 bucks. then you can have money towards a nice ssd.
that or you can go and update everything to te newer ivybridge or piledriver cpus etc. but who knows whats going to happen next year with the whole ddr4, may be worth it to build one then since intel will most likely be changing their sockets again by then and hopefully amd too
Instead of buying 8GB of 1600 RAM you could just buy another 4GB of 1333 to go with the 4Gb you already have. Save yourself $20 or so. The difference between 1333 and 1600 is barely noticeable. Quite frankly won't notice unless running benchmarks.
Unless you are intent on over-clocking it may be worth considering the i5-3350p. It's about $40 cheaper than it-3570k and only runs 0.3GHz lower. It can't be overclocked but will still greatly out perform an FX-6300 at stock. Once the FX is over-clocked they are probably quite similar, depending on how high you go.
Anyway that would take about $60 off your price so are around $390, less if you got for a H77 mobo since not over-clocking. $110 is enough for a half decent monitor
http://I don't think you can get away with a $500 upgrade and a Crossfire/SLI-ready Asus board and a monitor with a decent processor for under $500. Here's an alternative solution with a single GPU board:
Total is $503 for the upgrade shipped to your house. Just an option. This give you a great processor, a single-GPU motherboard, great RAM, a solid cooler and decent gaming monitor. You can upgrade your video card when necessary.