I have a wii which has around 90 games (give or take a few) and I am currently using a 160GB Harddrive with 30GB free. Depending on the games of course, and how many you have, 160-250 should be more than enough. I only play 5 of those games, the rest are just a collection
In regards to the ps3 side, I have 15 games downloaded (Also waiting on the 3.50 downgrader) which is 90gb. I would suggest maybe keeping your current harddrive for your wii, even though quite a lot would be wasted, and buy a 500gb-1TB Harddrive for your PS3, depending on the games you want
Also, from what I've experienced, the Wii harddrive resets to WBFS format everytime I want to add a game, so you may be able to put some movies or music on the side, just as backup for the rest of the space. Maybe making it into a new partition for those, with a NTFS/Fat format, haven't tried that.