Slow down there. From your previous posts, it sounds like this might be your first installation of an alternative OS, alternative meaning anything other than the Windows that came with your machine. I can tell you from experience that before you do any upgrade, no matter how "safe" it appears, you should verify a couple things.
First, make sure you have a backup of your OS for reinstalling, from scratch if necessary. Also be sure to backup any of your documents, save game files, media, that you just can't live without. I'd recommend picking up an external hard drive for the task if you don't already have one.
Second, realize that using your backup partition as a place to install Windows 7 could leave you SOL if you don't have any restore media. Some machine vendors (ie Dell) are sketchy on warranty and support, if you wipe it out you might be on your own.
Third, if you need a recommendation of partition sizes I'd go with 30GB - 40GB now-a-days for an OS partition. There is just too much bloat, and using Tools like Partition Magic to resize your existing partitions always introduces an element of risk, so you're better off padding it a bit. Just create another non-OS partition to put your games, media, etc... and the next time Windows goes down you can format the OS partition and reinstall like nothing happened. A word of warning on this is that some manufacturer's restore disks may enforce a specific setup, so if you don't have a proper Windows OS disk, this may not be an option. A second hard disk is always an option, assuming you set it as your primary disk.
Fourth, be ready for the unexpected. On a dual boot setup Windows 7 creates an additional partition for keeping track of boot records (typically 100MB in size). This partition isn't really dangerous, however it can be a real headache to get your other OS's working (read Linux). I'd recommend having another computer available at all times so you can do research if something goes wrong.
Finally, it is considered bad practice to upgrade or downgrade most OS's, especially Microsoft ones. Often not all of the files from the older OS are removed properly and cause strange behavior to occur. For stability reasons it is always better to do a fresh install, just to make sure you didn't have any strange hooks from previously installed programs. And if you didn't have any previously installed programs, doing a fresh install won't hurt anything anyways.
Good luck on the install and I wish you luck. I personally was impressed with Windows 7 64 bit until I found out that it has the same problems with my NVidia card as Vista 64 (when 3d mode is initialized some NVidia cards show garbage, no matter what drivers are installed). Everyone I know who hasn't encountered this issue says the RC is rock solid and is very pleased. Just be careful you are prepared for what you are getting yourself into.