RV630 won't simply be half an R600 chip. This chip won't have 32 Unified Shaders as is it should, as this would've been half an R600, insted it will end up with 24 of them. At the same time it will have 8 Texture memory Units, 24 Arithmetical logical Units and 4 ROPs (Raster Operation Units).
The limiting factor is certainly the 128 bit memory interface, but ATI just follows Nvidia and it wanted to make the chip more affordable and smaller. The worst thing is that RV630, Radeon HD 2600 series is delayed untill late June.
From what we know now, it should eventually end up being faster than the G84, Geforce 8600 GTS card.