If you want HD performance, but don't want to spend tons of €, then just add to your current setup and stripe it. I'm running 4 older WD raptors and it's quite fast. Yes, you can scale SSDs and get much better performance, but you need to get the right SSD HW to start and as we all know, it's not cheap.
I've got the same memory as you, though I'm running 12GB since I do a lot of stupid crap with video and Win7 was doing something stupid and eating all 6GB originally and killing my system.
As far as your GPU setup, in some cases, depending on the resolution, a 460 is not too far behind 285s. But 460s in SLI may not necessarily be worth the cost/performance you'll gain from your setup now, but it's your Euro. I just picked up two 460s, but I'm coming from two 8800GTs, so that's an obvious upgrade.
For audio? Eh...unless you're doing some audio creation, most on-board solutions are good enough. Save the space.
As far as your cooling solution, I wouldn't call that the best. The Prolimatech Megahalems is much better and a lot quieter. The memory comes with its own fan so it's kind of a moot point with that Cooler Master solution.