Some Socket 775 go up to Pentium 4 and some to a Quad core I could say the fastest Socket 775 is a Q9650 but will it work on your board...I don't know at this point.
You need to specify the motherboard model and bios version, then you can check the supported cpu's at the Gigabyte web site. Depending on which apps you are going to use a dual processor could be more cost-effective than a uad.
In the original P4, the Pentium 4 EE 3.46, this was quicker than the later P4 EE 3.73Ghz.
I assume it can take a Pentium D, in these the 965 was fastest.
Both of these chips are outclassed by the Core2 processors, with topping out around 3Ghz.