@kristian
my brother has a 9800gt with a duo 2.8
i have a q6600 at 2.4 with a HD4870 1gb, my system absolutly shits on his with arma, long story short yes u will notice considerable difference if u got the 4890. on the other hand i keep having this wonderful dream of playing arma maxed out without dropping under 60 frames very often. i am going to wait till about easter time and upgrade to a i5 and crossfire 2 5970's (dual gpu 5870 each card) and put the processor and gpus on water to overclock it all, i say easter because the price of all that will have dropped heaps and also hopfully by then arma will be fixed with the release of operation arrowhead. oo i almost forgot to mension the SSD drive, sposed to make arma run heaps faster
EDIT:@fieldmarshalzhukov
q6600 (stock 2.4)
hd48701gb
corsair xms 3x2gb 1600mhz ram
gigabyte EP4T-UD3LR mobo
normal hard drive
i used to run my processor at 3.6 and also overclocked my 4870 a lil and the performance was very satisfactory, i can pretty much run the game maxed out except for Antialiasing needs to be normal and view distance not very high (around 3k), and still get around 30 fps when there is shit going on. im running stock at the moment because i am about to format and put on win7
have a look into SSD drives, its another form of harddrive that is alot faster, its pretty new so might be abit expensive still.