There's no way to do this using Overture. Not positive about Google, but I've always *assumed* their data is accumulative. And of course that wouldn't really help you out anyway since they don't give exact numbers. WordTracker, anyone? I don't use it, but plenty of people here do. Sorry I couldn't be more help, MrSpeed.
Keyword Discovery.com is a new database by Trellian
It offers search term suggestion, related keywords and misspellings. Keyword Discovery also has a search trends feature, this means you can find out what people are likely to search for over Christmas and the next Valentines Day months in advance.
Most other search term suggestion tools rely on the past month's data, so by the time you find out what keywords you need to target for Valentines, it's all over.