With those prebuilt machines you could upgrade them up to a point, but the power supply usually is the first part to stop you.
I would say that you should build a new machine, if you can afford one. That would get you the speed you are looking for.
You can get a decent machine for $500 these days and then add $99 (unless there are sales) for Win7 home premium 64-bit.
With general specs:
Dual, Triple or Quad core cpu
motherboard with Sata 6Gb/s & USB3 support.
4GB of ram
low end gaming graphics card ( 5670/5770/6770, GTS450/GTX550Ti/GTX460)
Low end case
quality PSU from antec,corsair, seasonic or XFX
DVD writer
1 TB HDD