Alright. Sony Vegas Pro is all about the processor. Period. 4GB of RAM should be fine, unless you're running Photoshop or any other Adobe products.
For video editing with Vegas Pro, I wouldn't recommend anything slower than the fastest i7 processor you can afford... So with a 2009 Dell, odds are you'll need a new computer. Your motherboard may or may not be compatible with the new procs, as well as any other updates you'd need to make them run happy.
So, what kind of budget do you have? I can make a recommendation or two if you want. I've been using Sony Vegas Pro for a couple of years, on semi-custom i7 system and it's very nice...