Old Setup with BF3

On a old CRT 15 inch monitor you are going to be able to max out everything .

on a 30 inch hi res its going to be a crappy slide show

what else would you like to know?

Haha what about a 22'' Dell monitor with 1920x1080 dropped to 1680x1050?

You'd be better off playing at native resolution and dropping quality settings till you get playable frame rates

I still uses a C2D cpu for gaming . I just added a 6870 and OC the processor to 3.6 GHz . At 1080p its running the original Crysis game at 45 - 50 fps on high
The higher the resolution and the higher detail in a game the more its limited by the gpu and not the cpu .
If I was you and wanting to upgrade cheaply I'd just replace the graphics card . The 9800 x2 is pretty old now
So whats your budget?

and can you overclock the e8400? Does it have the stock cooler or something else?

The e8400 is stock and can most likely be overclocked, but I'm not too familiar with how to do it (I can probably call up one of my buddies and ask him). I'd say my budget is $200-$300, but I'm working to increase that   Thanks a lot for your advice.

There used to thread dedicated to how to overclock a C2D in the overclocking section , but its gone now . Im sure there are a lot of online resources . If you have the stock cooler then upgrading that makes sense IF you can reuse it on your next computer .
No point spending $50 on something that you cant re -use when you update

Heres what I would do on a $300 budget

Scythe SCMG-3000 120mm Heat Pipe CPU Cooler
http://www.newegg.com/Product/Product.aspx?Item=N82E168... $50

EVGA SuperClocked 01G-P3-1567-KR GeForce GTX 560 Ti $220 [ after rebate ]
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

If you decide to change your cpu, mb , RAM you should be able to use both of those in a new computer
A couple of other things to consider :
your power supply had better be up to it . Probably it will since the 9800 X2 probably uses more power than the 560 ti any way
With the Mugen you should easily reach 4 GHz , even with the power saving features still on , but this will depend on your motherboard . Watch your temperatures

^^
+1