I overclocked my videocard (HD 3650, DDR2 512mb, 725MHz, 800MHz(DDR) memory).
I think it's a good card, maximum values was: 904.5MHz and 1098MHz(DDR) memory. Without any manipulations with voltage and without adding any coolers on the memory chips. Core was overclocked by 24.7%, memory 37.3%.
Here 3DMark's results. I also overclocked cpu (Athlon 64 3200+, 2000MHz) to 2500 MHz with voltage increasing from 1.4 to 1.55 volts. Here is the comparison of default config and full overclock:
2006: 3022 -> 3989. Gain 32%.
2005: 6735 -> 8895. Gain 32%.
2003: 9304 -> 12323. Gain 32.4%.
And GTA 4 benchmark (with latest 1.0.7.0 patch):
default config: 13.03 FPS
with overclocked videocard: 14.40 FPS
with overclocked videocard and cpu: 17.18 FPS - Gain 31.8%
I think it is a good result.