Incremental System Upgrade.. should i bother with the card yet?

[miseryguts]

OK so I'm just about to purchase a new cooler (Noctua NH-D14) and an extra 6Gb..? (undecided yet waiting on some feedback in the memory section) of memory.
I currently have a HD 5870 at stock speed most things run fine at high/max detail 1920x1200 but some recent games are causing system reboots.. Battlefield 3, Skyrim, Darkness II and a few others however i have been reliably informed that this is much more likely an issue with my processor i7 920 (3.45 O/C with a stock cooler which is why the Noctua) ..apologies going off topic..

Is it worth a couple of hundred quid (can't afford any of the top end cards) to upgrade my card now & how much of a performance gain could i expect in that price range or is it worth waiting till the end of the year or even into the next until Iv'e more money & there's more choice..?

Thanks for listening.

 

[miseryguts]

my stages are this:
* Buy a decent card and use it until games demand more than it can give
* get a second card and sli / crossfire for that little boost (by then the cards are old and cheap to get the 2nd card)
* once those twin cards are not juicy enough, i overclock the nuts out of em and flash them to that overclocked state

I hadn't really considered another 5870, but depending on the performance gains I would certainly be interested.. but don't really want to have to upgrade my power supply (850w) in order to support it, Iv'e also read that plenty of games still don't support crossfire setups.
Iv'e never heard of flashing cards to an overclocked state before, what does that involve & how does it benefit you..?