What I think:
CPU - Good idea to upgrade, that E6750's looking a bit old. Your motherboard does support the 45nm Core™ 2 series, and a Q9550 looks like a good option. Remember though, you need to flash your BIOS version to F9 in order for the motherboard to support it. If you don't want to flash your BIOS, you could still use a Core™ 2 Quad Q6600 2.4GHz without the need to flash your BIOS - though you'd have to get one of these CPUs from eBay. I suggest you get the Q9550 though, it's just another option.
RAM - 4GB of DDR2 is more than enough for gaming, it's a decent amount.
Graphics Card - GTX 285 looks good, will perform a bit worse than an HD 5850 or approximately the same as 8800GTS 512MB in SLI.
PSU - The GTX 285 is quite power-hungry, and I believe you should upgrade your PSU to something a bit beefier, although 500W is adequate. At Hothardware, their Core™ 2 Extreme QX9770 system with a GTX 285 only took 331W, but they seemed 50W-100W lower than other sites, so beware of that. The current that the PSU provides is more important than the wattage though nowadays. I'd go for a high quality 550W+ power supply with two 6-pin PCI-e connectors. A Corsair HX650W would do the job well, it's modular too, and give you some headroom for overclocking. If you want to get a cheaper PSU, an OCZ ModXStream 600W should work too, although OCZ's PSU reliability and quality isn't as good as Corsair's.