Nvidia Geforce 7800 GT - 256MB GPU:440MHZ, MEM CLOCK=1400 GDDR3
Geforce 7800 GT and GTX are only available in PCI express.
The only Geforce 7800 AGP card made by Nvidia is the GS.
A Geforce 7800 AGP card advertised as GT, GTX, or XT is a card that has the ROM edited, and most likely has used the ROM from a PCIe card to report as such.
As for speed ratings, Geforce cards can be overclocked, some better than others. There is overclocking and superclocking. The card starts as 375mhz w/1200mhz memory clock. Some third party producers of the 7800 GS (BFG for one), using the Nvidia GPU, often sell a 400mhz/1300mhz OC, or over clocked version. Some flashers of Geforce cards superclock these to bring a better sale price. Any rated at higher than 400mhz/1300mhz are considered to be superclocked.
Of course, any overclocked or superclocked card will require more power, produce more heat, and have more stringent cooling requirements.
One thing that over clocking or superclocking *doesn't do*, is unlock pixel pipes to allow the GS (16 pixel pipes) to "become" a GT (20 pixel pipes), GTX (24 pixel pipes), or XT (probably a GTX edited ROM).
Whatever the report of a flashed Geforce 7800 AGP card, it is a GS.