No, your system is fantastic! 990X CPU is still one of the best performing CPUs to date. Just, in most applications a SB CPU will beat it, but it applications that will eat 6 cores, it is a monster. 2 hd6990s is basically the best GPU set up there is, and 12gb DDR3 RAM is very high-end. You are good to go for a few mores years.
By the way, if you want more power, you could always overclock it. They hit 4Ghz pretty easily and can hit 4.5Ghz sometimes I think. Although, the 990x is clocked in at 3.45Ghz already I think, which is still powerful.
No reason to upgrade whatsoever. You can get a slightly faster CPU these days but you won't notice the difference. My rule of thumb is to upgrade every other generation and to always buy the "low end of the high end". Ie when I put my current rig together in 2009, that meant a core i7 920, upgraded since with 2*560 Ti. I plan to skip sandybridge and upgrade again some time in 2012.
In your case you already have the "high end of the high end" from the previous generation of i7, so getting a very small performance boost will be a very expensive business. I'd leave it 2 years before considering an upgrade.
your system has plenty of grunt. it doesnt need to be overclocked and your gfx cards will kill any game out atm. your system is pretty much top draw and for some1 who claims to be out of the loop you managed to put together a little beauty. yes you could upgrade to the new socket 2011 and get a sandy bridge six core and you would get a noticeable increase in productivity performance. but gaming wise there would be no difference because your gfx cards dont have enough grunt to bother either of them to cpu's so they would both max the cards and give exactly the same fps.
then its down to cost. will the next gen cpu offer enough of an upgrade to be worth the cost. the short answer for you is no. you dont do heavy cad so you wouldnt benefit from the extra horsepower. infact the system you have is way way overkill for what you use it for. gaming it will max out everything for the next 2 years at least.