These are the only two cpu's that you can upgrade to with the LGA 775 socket MB that you have.
Your other option is to see if a standard atx motherboard will fit into that Dell case you have so that you could upgrade the cpu , MB and ram.
And you could just reuse the dell power supply and hard drive, and WALA! A brand new spanking fast computer. Don't bother upgrading the CPU, or putting anything different in that crappy case, if you were to reuse the power supply, I would think of getting a new one in the future, since dell power supplies aren't exactly that powerful and top-of-the-line.