Upgrade MacBook Pro vs replace?

Hello

I currently have a 2009 unibody MBP C2D 2.53ghz, 4gbs ram, 250gb hd. It is a bit long in the tooth now and running fairly slow.

I am either looking at replacing it with a MBA or a new 13" pro if they come out! The other option is to upgrade it with an ssd, optibay hdd and 8gbs of ram for around £170.

How much faster would the machine be with those upgrades? Or is it just more cost effective in the long run to get a new machine? I do love the screen size even if it is a bit heavy!

I mainly use the machine for editing RAW files in Aperture ands photoshop (cs4 at the moment so the extra ram won't help)

I'm also concerned that the machine is nearly 3 years old and may start to see applications requiring higher specs, do you think that will be a problem?

Thanks
Chris