My computer is a similarly configured machine (quad core
[email protected], 8GB DDR2, Geforce 9800GT, 1TB+750GB drive, etc., going on two years since I built it, I think)
Personally I've not even thought of ever upgrading it. It still runs anything I throw at it (stuff like 3ds max, Pinnacle studio 15, Flash CS5, Paint Shop Pro, Visual Studio, Eclipse, FL Studio, FamTracker etc. all running at the same time) and games like GTA4, Just Cause 2, Crysis, etc run without any issues. The only problems I've had so far is that the Q8200 doesn't actually have any virtualization hardware support, so I cannot run OS/2 as a guest OS in most Virtual Machine programs.
I think a good point might be if you have to ask if it's worth it, it's probably not; but if you want it, that can be reason enough.
To be honest, as far as I'm concerned, there really is no such thing as a "crappy PC", generally speaking; after all, even 286 machines still find their use as embedded systems; of course crappy for a given task is definitely the case; a good gaming machine in 1999 might have been a Pentium 3 with 512MB of RAM, but nowadays people seem to think that is useless for anything, but that is simply not the case.
We have to remember, to only reason we ever think our computers are "crappy" is a result of seeing "better" machines in commercials/magazines or hearing about them; I was literally using a 350Mhz machine for 5-6 years leading into 2007 or so and as long as I understood it's limitations I didn't have any problems. When I finally upgraded, what I was finally able to do that I hadn't even considered previously was overwhelming.
