The minimum power requirements of the 5770 and 5850 are 450w and 500w, respectively. You prett much have your PSU maxed out with the 9800GT. If you want to get a better graphics card now I suggest getting a new PSU now that you can also move to your next rig.
The E8600 is a fast chip and I see you have OC. The bad news is this really means you probably don't have overhead on the current PSU to increase the video card. Is your video card OC too?
The good news is the E8600 - especially with OC - is a very fast chip and can easily handle a graphics card upgrade.
Is the upgrade worth it? Well that depends on what you are missing now that the upgrade will provide, how signficant that is to you, and what is the oppotunity cost of the cash - that is what will you be giving up to get this and how important is that to you.