A simple upgrade of your video card will make all the difference as it did for me. Now granted your old motherboard can't take full advantage of it, but the performance boost will be great on gaming.
In addition, you can always use the new card in a new system if you eventually go that route, so the investment is worth it. Right now in regards to gaming, that is the biggest single upgrade for you.
Don't forget many video transcoding programs and photoshop can take advantage of the video card. In an ideal situation, you'd want to start from scratch, but when affordability is the case, one can do a lot with a small upgrade.
Here is a Passmark benchmark chart for it at stock:
http://www.cpubenchmark.net/cpu_lookup. ... +@+2.40GHz
It is near the bottom. You can clearly tell how much it shows its age. They were put out in early 2007 after all, but it is still quite impressive how well it does, five years later.
I've wanted to step up my desktop from a Core 2 Duo to a Quad, but the prices are quite ridiculous in comparison. When people want more money for one than a Core i5 which outperforms it in every way, I find it quite dumb. I would never be able to justify paying that much, I'd rather do a full overall first. I know supply and demand, but it is pointless if you end up sitting on it forever.
When all else fails, replace the user.