Replace iMac or upgrade it?

Hi!

I'm in a big dilemma right now.
I'm pretty tight on budget but my iMac started being kinda slow, I can't multitask through heavier software like Adobe and Logic Pro which I use a lot.
Also my wireless mighty mouse is becoming a pain and the scrolling ball barely works anymore no matter what I do to fix it.

My iMac specs are:
24" Aluminum (mid 2008)
2.8Ghz C2D Processor
2GB Ram
320GB Harddrive
1.5TB External hard drive (so no problems with storage whatsoever).

My question is, since I'm pretty tight on budget, should I upgrade the RAM to 4GB + buy a new mouse (which should be like $200)? or should I wait another 6 months (the least) as I am right now and just buy a brand new 27" iMac?
It's just that I'm not sure when my iMac will be ready to retire, I still think it's in great shape, just needs an upgrade.

Thanks

I would try the ram upgrade I have noticed things are requiring more ram every year and a 2.8GHz C2D is still respectable besides it will help with resale when the time comes.

Upgrade the RAM and continue to save money for a new Mac. The one you have now is pretty good actually.

If my processor is good enough as you say, than I'm definitely gonna just upgrade the RAM to 4GB and wait another 6-7 months 'till I buy a new iMac =]
Thanks guys for the replies you helped me a lot! =]

I have another question please.
My iMac is certified to receive maximum of 4GB RAM but I read that it actually could take up to 6GB unofficially and it would still be stable, I read it here:
iMac 2007 and 2008 can be upgraded to 6GB RAM regardless of specified 4GB limit
Is this true? will it be stable? or should I just stick to the 4GB program and not look for adventures?
Thanks =]