This is one of those questions where someone can't tell you what you need because you are looking for something that fits your own personal taste.....
So, few questions.
Are all the programs you are using performing fine?
Do you find yourself waiting for things to render in PS?
Do you want to run the latest games?
If you say, yes/maybe to any of those questions then you should think about upgradeing.
Then you have to decide on how much you want to spend.
Examples would be:
$1000? Build a new system.
$500(or less)? Vid card, and another stick of ram(for 4gb) This would be the most bang for the buck in my opinion if you decide not to build another system(along with ur hard drive upgrade)
$250? Stick of Ram for 4gb(along with ur hard drive upgrade)
This is just examples of course, its all up to you dude lol