As I said above, the plain x800 is a better performer than the x800GT. that's why it costs more.
The GT is a slightly crippled version of the x800. the x800GT can't do as much with 400Mhz as a 'normal' x800 can. The x800 can process around 50% more per Mhz than the x800GT, because it has 12 pixel pipelines instead of the GT's 8.
Think of it as an engine's max RPM (Mhz) Vs. Horsepower/Torque.(Pipelines). Imagine the GT is a 2 litre 4 cylinder engine, then the plain x800 is effectively the same engine but with 2 more identical cylinders, so it's a 6-cylinder 3 litre engine instead... So even if you get the 2 litre to rev up to 7000RPM and the 3 litre to 6200RPM, the 3 litre's still going to have more power.
Just forget clockspeed when it comes to graphics cards. It means nothing when comparing a bunch of stock (i.e. not overclocked) cards.
The x800 will give better gaming performance than the x800GT, which is why it costs more.