I have been thinking about upgrading my home PC which is approx 5 years old now. I am using it mostly for browsing and office work but I should be able to game atleast casual at it. Now I have:
Windows Vista Home premium
Intel Core 2 Quad CPU Q9550 @2.83GHz
RAM Memory: 3GB
Nvidia Geforce GTX 260 2GB
I can still play games with those in low/medium settings yet I seek to speed things up a little.
Now my question is, what are the most useful upgrades without me having to pay too much? (Because otherwise I'd most likely have bought a new system) And is it still worth upgrading?
You will be looking for a whole lot of changes to 'speed things up a little'.
Most all of your tech is a bit on the aged side of the equation - but some of it can still be used - at least for the interum.
PSUs have been fairly stable in their construction for the last 10 years - I.E. standard pin-outs and such, though the prevelance of SATA power connections may be the most notable shift. Depending on the 12V rail on your PSU - you should be fine in keeping that. Check out the PSU Tiers on Toms to see where your brand is located. Aim for at least a tier 2 if you need to purchase.
The GTX260 should be fine for a while - though if you really want to - you can replace it for better technology.
My suggestion for you would be to replace the motherboard, the CPU and the memory and see how your gaming improves. The next step would be to replace the video card - and possibly migrate to a SSD for your C: drive.
You should also upgrade your OS to something like Windows 7. I did a recent build of windows 8 - and I am still getting used to it so I can't reccomend it yet.
With the MB, CPU, Memory and OS - you are looking at a fair chunk of change. I would figure around $600. Although you might be tempted to just buy an entire new system, the benefit of building your own is that you are able to get the quality components that will result in better performance and longer life (depending on your desire for overclocking.)