I would say it depends on how you're planning to set up your laptop. If the SSD is going to be your one and only drive and you're going to be using it for data storage as well as booting, then I don't think even a 128GB would be enough. But if you're going to use the SSD as the boot drive and perhaps a few really necessary programs like an anti-virus suite, etc and use the original 320GB HDD or some other external HDD for storage, then I'd say the 64GB SSD should be fine. But if you can comfortably afford a larger SSD, then that would work, too.
Here are some tutorials that you might find interesting: SSD - Install and Transfer the Operating System SSD / HDD : Optimize for Windows Reinstallation SSD Alignment SSD Tweaks and Optimizations in Windows 7
As far as using 3rd party tools to maintain Windows 7, I think many people recommend NOT using them (with the exception, perhaps, of CCleaner.) Windows 7 built in tools like Disk Cleanup, Check Disk, etc are pretty much all that's needed because 7 isn't anything like XP that had to be tweaked and adjusted for optimum performance. Just my opinion and others may very well disagree.