HD6850 (i3-2100 vs i5-2400)

http://www.anandtech.com/bench/Product/289?vs=363

You'll see some gain on the 2400. Not sure what GPU they used in those benches, but at the very least you probably won't see any bottlenecking from the processor side of things.

You'll will see noticeable gain with i5 2400.If you want to upgrade get a i5 2500k..

ya get the i5 2500k bcz it is overclockable if u want..it is the best i5 processor and i think this is the best cpu for the gaming with any gpu

I have an i3-2100 and a mid-ranged GPU, as well as a motherboard that wont CPU overclock.....so I dont know why you automatically change the subject to the 2500k ...hmm?

i don't know what game(s) your running but i can tell you my i3 2120 does fine with my 550ti. (a little weaker than your card though)

i just ran some benches in COD MW3 for my little experiment of undervolting my cpu for better/lower power consumption because, well thats all i can do for geek fun with it.
http://www.tomshardware.com/forum/327800-28-undervoltin...

bottom line is at no time did i get the "feeling" my cpu was causing a problem. actually the highest load on my gpu was 67% and cpu was 25ish%. and the times my cpu had a load is when the game/map was loading.

i am starting to think a sli or Xfire configuration is what would cause a bottleneck.
but would an i5 improve game play? i would think so.

Seems like you should do your testing with something more demanding if your GPU and CPU are barely stressed?

The i3-2100 should be fine for the HD 6850. Most games only use 2 cores, however there are a few games that do benefit from 4 cores. If you know you are going to be buying a game that can use 4 cores then you may want to opt for the i5-2400.

You could also opt for the slightly faster i3-2120 CPU instead if you want to keep costs down. Newegg is currently selling the i3-2120 for $128, plus if you enter the promo code EMCNGND32 that reduces the price by $15 down to $113, but it expires on March 12th.

http://www.newegg.com/Product/ProductList.aspx?Submit=E...

The i3-2100 is currently selling for $125.

http://www.newegg.com/Product/Product.aspx?Item=N82E168...

The one thing that a core i5 has that the core i3 doesn't have (other than 2 extra cores) is Turbo Boost which is an automatic overclock as long as temps are not very high.

The extra 200MHz of the i3-2120 compared to the i3-2100 should help make up for the lack of Turbo Boost and lower price should help sweeten the deal.

your absolutely correct. BFBC2 did just that  
http://www.tomshardware.com/forum/327800-28-undervoltin...

99% on the gpu and no more than 50% on the cpu except when the game was loading.

i need a bigger card.  

and to the OP.

no matter what anyone else chooses to believe, if your gpu is running 100% and your cpu isn't sweating, you need to upgrade your GPU for a performance increase worth spending money on; upgrading the cpu would NOT be cost effective.

yeah i changed my mind since last post but not really.