Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
Depends what you do. Games are still mostly 32-bit so extra RAM won't help with them but if you're multitasking or running Windows in VM, then more RAM will definitely help.

For 50 pounds, I would go for it.
 
Currently have 4GB of ram in the iMac.

What would 8GB do with regards to performance within OSX and Win7 64Bit

The iMac is a i3 3.2GHZ

I play SC2 and TF2 in OSX and Win7.

I can get this http://www.crucial.com/uk/store/mpartspecs.aspx?mtbpoid=F0FF4789A5CA7304

so i'll keep the 2 x 2GB already in the machine and just add another 2 x 2GB sticks for £50

Or am I wasting my money ?

In my opinion adding RAM is one of the best upgrades for the money there is. However without knowing how you use your iMac it's a little difficult to say. I am guessing you are using Boot Camp rather than virtualization software however either way I would recommend upgrading your RAM.
 
Yeah I do run virtual box and sometimes a network emualtor like GNS3

So yeah might got for it

Just wondering if OSX will run better
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.