It IS your CPU mate, It's a dual core pentium 4 which was weak compared to AMD's offering at the time.
Regarding vsync, that's what vsync does. If you can't hit 60fps it'll drop to 30 until the hardware is able to render 60fps again.
To get any more FPS, you are going to need an upgrade, which is not possible due to your computer being a Dell. The board won't support Core 2 Duo CPU's which in itself would give you a decent boost, because the architecture is much superior to P4, the case and motherboard are likely non-standard ATX and the powersupply will be inadequate.
So you can:
A) Take the plunge and spend a few hundred on a new gaming computer
B) Lower your settings and deal with it
C) Keep ignoring what people tell you and get nowhere.
Even with a budget of $400-$500, you can reuse your graphics card as it's still okay for most games when paired with a nice CPU. $500 build:
CPU:
http://www.newegg.com/Product/Product.aspx?Item=N82E16819115072
Mobo:
http://www.newegg.com/Product/Product.aspx?Item=N82E16813157249
RAM:
http://www.newegg.com/Product/Product.aspx?Item=N82E16820231277
GPU: Re-use your GTS 250
HDD:
http://www.newegg.com/Product/Product.aspx?Item=N82E16822152185
PSU:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817207012
Case:
http://www.newegg.com/Product/Product.aspx?Item=N82E16811119195
Total Cost: $510.94 before shipping/rebates - not bad for a gaming capable system, and an upgradeable system at that. I'd normally chuck an AMD CPU and Mobo in there to keep price down, but I don't play SC2, so I can't say how they perform.
Bottom line is that your system is the bottleneck. As for people having issues on i7's, are they running the latest patch? You'll probably find issues are from a while back and aren't up-to-date.
And building a system is a trivial task, see here:
http://www.computerforum.com/104641-how-build-computer-step-step-photos.html