"...but there was no point increasing it to a total of 4GB as vista only 'sees' 3.2GB of ram"
It's true that 32-bit versions of both Vista and XP cannot see all the RAM (due to some of the address space being used by hardware.) In some cases of older hardware designs, the motherboard may limit the amount even when using 64-bit processors and OSs.
If the graphics use system RAM (as is the case with many laptops) - then this RAM is also "lost" to the graphics.
It's never exactly 3.2GB - it depends on the exact hardware and graphics configuration.
If the RAM is dual-ported - then it may be worth fitting 4GB to get the extra speed that dual-porting gives.