Upgrading from a HD4890 - £150 to spend

[Josh P]

If you're playing Arma 2, a couple of things to bear in mind; as said above, it is a CPU hog so overclock the nuts out of it. Secondly, there seem to be a lot of threads on the Bohemia Interactive forums about problems with Arma 2 and the 6xxx series, namely texture issues and crossfire scaling. The review from hardware.fr (the only site I can find that regularly benches Arma 2) had the 69xx performing below the 6870!

I am currently running a 5770, which as far as I know will perform similarly to your 4890. My chosen upgrade is likely to be a GTX 570. As this is beyond your budget, you could perhaps try a GTX 460 now, and then get another in the future if your motherboard supports SLI?

 

[OpenToSuggestions]

I honestly don't care how hot my 6950 is. The fan never goes above about 35% anyway but if it did get hotter then the fan would spin more quickly.

 

[bru]

ignoring all the petty bickering and the my card runs hot, oh mine doesn't chatter.

yes you will see a marked decrease in frame rate if you get a monitor with a higher resolution.
personally seeing as by your own admission your running your currant system at stock i would disregard any thought of buying a 6950 with the sole intention of flashing it, as this is not a performance upgrade i would recommend to just anyone, its a goodly step up from just overclocking, and you haven't taken that one yet, so in my opinion avoid.
in your situation i would be after a better monitor first, as you can always run games with lower details on a slower card but in certain circumstances what you gain resolution can help offset the lowering of the detail levels.

of course i would read up on the over clocking of your currant system as this can also help reduce any performance deficit when moving to a higher resolution monitor.