I checked, the 520Mhz PXA270 has the same 208Mhz system bus freq as a 624Mhz or 312Mhz.
I think the slower speed is due to the VGA display (and the absence of 2700G chip). The PXA270 has only 256KB internal SRAM. This is enough for the QVGA video memory for X30. It's 950 FPS GAPI speed is even faster as with system memory (like QVGA Asus A620). But for VGA you have to use system memory, external SRAM or graphic chip (like 2700G) for the frame buffer. I'am not sure about A730 yet, but from the memory speed it seems the overal slower benchmark results are due to higher system bus load. If the LCD is controlled by the PXA270, there is plenty of data to be transfered all the time when the VGA LCD is on.
So this won't be a "Gamer" PDA, but really hope to be wrong. And because i didn't hear any '3D acceleration' promo for the Ipaq Hx4700 either, i think it will has the same problem (with a little more CPU power).
ps: i think there are smart LCD panels on the market with own video memory. but i don't think this is the case here (probably not cheap)
ps2: sorry about the technical stuff

i just want to buy a PXA270 device this summer for WMMX development. possibly with many new features for compability testing.