Well, speed it up in what? Overall speed, general tasks, rendering, gaming, photoshopping?
You have plenty of RAM for what most people use. However, if you have a 32-Bit OS installed, then you won't have access to all 4 GBs. Upgrade to a 64-Bit version of windows (if thats the OS you use). That way all 4 gigs can be utilized.
Your processor is fine, but you can't overclock it due to it being a Dell. I have a 3.0 Ghz core 2 duo, and I multitask like a mother, and it works fine for me.
The problem with gaming side of performance increases, is that you have a crappy PSU, which will severly limit what GPU you can get. I'll let someone else find a compatible GPU that will work with that PSU.
Also, make sure you defrag the HDD once in a while. If you have never done it, it can slow your computer down as well. Make sure you have a good anti-virus program that you can shut down. This reason is because an active anti-virus is really good, but can slow your computer down a great deal. If you shut it down during normal use, and start it up when you want to scan, that works.
You can also go into msconfig and tell both cores to be used during computer start-up, as well as disabling what programs are automatically started up by your computer. A decent guide is here: http://netsquirrel.com/msconfig/msconfig_vista.html . The process is very close to the same in Windows 7 and in XP.