I'd say wait until the 5850's are readily available- and get a stock OC'd version (1gb gddr5, maybe?) w/ a non-ref cooler. Yeh, you could overclock yourself, but sometimes they specially pick the most responsive chips- so u get a leg up, and a bit more headroom too- plus u don't need fuzzed software running in BG keeping clock setting going while preventing a house fire. The non-ref, stock, cooler is to not go deaf while keeping it cool- and to cover your ass (no getting a 3rd party cooler + losing warranty coverage).
Don't do anything until then, except get real familiar with the in's and out's of your system, if you're not already; such as overclocking. You said you'll likely be adding win7 in Oct, from what I've read the 5850 won't be at the stage I suggested as your purchase pt until mid-end Oct. So plan to update the graphics when you get the new OS. Waiting until that time won't make the core2 lineup dry up, get scarce, and go up in price... it will only get cheaper within the month. Come back, and ask again when you're about ready to get win7 and the 5850. The core2, i5, and i7 prices should all be more enticing by then- so you'll end up ahead compared to if you buy now- just suspend ur credit card in a block of ice, and tac ur goals to the ice! haha.
For gaming, you'd likely do better getting a higher clocked dual core than a quad (in the core2 realm). Until prices come into focus on how intel's three socket platforms are going to price out by year's end, I'd say getting an i5 or i7 is a lot of buck for not much bang- unless you pick up an addiction to folding, digital content creation, or start going manic on multitasking.
See if you can get the E6400 OC'd to ~3.3-3.5ghz. Seems doable on 1.4V [someone check me on this: this is safe for the 65nm core2s, right?] from a quick google of 'E6400 typical OC.' But don't go frying the thing and force your own hand. What hsf do you have on the cpu? Might want to ask around and find out what a good hsf is that can readily fit all thre intel sockets- u could get it now, and use it no matter what you decide to do. If you're sure the E6400 won't be in your system come November, maybe even consider lapping it if you have the time + patience.
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