Upgrade 2007 iMac or get a new iMac vs Mini + Monitor combination

[ozone]

I've got an old iMac (circa 2007) - still runs fine, but is quite slow now. It's a work computer, so I don't run anything terribly demanding, but things that used to run fast are now slowing down noticeably.

It still only has 2 GB of RAM: I tried several brands of RAM chips locally - none worked. I was considering just ordering the 6 GB RAM upgrade from OWC. Does anybody know just "how much" improvement I will see?

My other option is to just get a new 27" iMac, or if shipping times are just too long, possibly a Mac Mini + 27" monitor (either Dell or Apple branded).

I'm conscious that this computer is now heading into its sixth year of continuous use. I try to replace my hardware just before I need to (that is, before things fail).

Any thoughts on this? Thanks in advance.