what is long in the tooth? is it cache related?Athlon 64 X2s are getting a bit long in the tooth, might get slight bottleneck but overall for COD it should be fine, there should be no reason that you couldn't play COD with high detail @ 1440x900 with 2xAA. Saying that though COD is on of the easier games as far as specification most other games you'd probably have to scale back to medium settings.
The GTS 240 would be way better than your current 9400 GT but the GT240 is getting long in the tooth as well, I believe its a rebadged 9600GT from 2008 but its still capable. If the budget allows I would drop in an Athlon II X3 and a GTS 250 which should be attainable for less than $200.
But to answer your question more clearly, yes COD will play well.
lol.
Here is the golden rules, always read the reviews regardless of the perceived hierarchy.
Most desktop Nvidia video cards are "Geforce" they just stopped announcing it and marketing it as a "Geforce". The 9800 GT is faster than ALOT of video cards with GT infront of its name. For example the GT210, GT220, GT240 are all slower than the a vanilla 9800 GT.
i know nothing about graphic cards cpu requirements this is my proccessor ADA5600IAA6CZVanilla, I mean its a regular 9800 GT, nothing fancy not an overclocked version or a soup up version.
The 9800 GT from 2008 is a rebadged 8800 GT from 2006 and is still faster than the GT210, GT220, and GT240 from late 2009. Shocking isnt it!
Edit:
I'm not saying to suddenly get the 9800 GT, you need to decide what your budget is, whether your PSU can cope with it before you jump and buy it.
What model name/number is your mates quad core? Not all quad cores are the same.
Even the slowest quadcore would most probably be faster than the fastest dual core "overall", there might be the odd task the dual core would be faster in. More applications are becoming multi threaded so dual cores will be going slower whilst quads start to peek in performance over time. Think of a quad core as wine, it matures with age.
Not true, for the most part during 2009 dual cores were starting to show its age. For example with clock for clock the old Athlon X2s are slower than the new Athlon II and Phenom II quad cores so even if the application or game task you are doing only takes advantage of two of the four cores the "new" quad cores two cores are faster than your old athlon's two cores.
Alslo.
Palit GTS 250 $112
http://www.newegg.com/Product/Product.aspx?Item=N82E16814261051&cm_re=gts_250-_-14-261-051-_-Product
The performance of all these cards can be found on the chart above.