^ Not quite. The only difference between the RS780(3xxx) and RS880(4xxx) IGP's is that the 4xxx ones have UVD2/ATI Stream and DX10.1. Other than that they're all the same core. Here's a rundown:
350MHz-
HD3000(760G)
HD3100(780V)
500MHz-
HD3200(780G/780E) (adds UVD, 8x AA)
HD4200(785G) (adds UVD2/DX10.1)
700MHz-
HD3300(790GX)
HD4280(890GX) (Can Hybrid with the HD4350 somehow)
There are also discrete cards that use the same core:
HD2350- 525Mhz core/400Mhz Memory
HD2400 Pro- 525MHz core/400MHz Memory (Hybrid Capable)
HD2400 XT- 700MHz Core/800MHz Memory
HD3450- 600Mhz Core/500MHz Memory (Hybrid capable)
HD3470- 800MHz Core/950MHz Memory (Hybrid Capable)
I should note that the IGP's tend to overclock very well. People have gotten the 785G to 1GHz, though on air 800MHz is more reasonable. Also, my brother managed to play through Crysis on a 3450, albeit on low settings...so they can handle decent gaming in a pinch. An overclocked 785G and a 3470 in hybrid actually wouldn't be too bad.