Some say 4GB is enough or 6GB (triple channel memory setups on the old i5's and i7's). By now the 64-bit technology is more mature so there wont be any problems and especially if you get windows 7 Pro 64-bit has a compatible mode of running things that arent compatible with Win 7. Just moved to 64-bit came form 32bit Vista, and the only issue I found that there is no linksys drivers for my network card. But theones that windows assigns work great.
So if you get the minimum 4GB with a 32-bit OS, you will only use 3.1GB, get the 8GB and get over with it. It will leave you headroom, and you will multitask better. 8GB is becoming more standard now, 12GB or 16GB would be overkill.