one correction: N900 and iphone 3gs don't have same gpu. N900 has SGX530 Iphone 3gs has SGX535 http://en.wikipedia.org/wiki/PowerVR
Although the 3GS has a superior graphics card in comparison with the N900, it's underclocked to one forth (!) of it's intented speed, due battery and overheating problems of the Iphone 3GS. This means the N900's PowerVR SGX530 is actually twice as snappy (no pun intended to Snapdragon) as the 3GS' superior SGX535 chip. As for raw numbers, the N900's graphics card works up to 14MPolys/s, while the 3GS is theoretically supposed to be able to reach 28MPolys/s, but as described above, is underclocked to only 7MPolys/s, which makes it somewhat inferior.