Not sure what you mean by 'access' capacity but about the only advantage partition a drive has is making it a bit easier to do backups (drive images) and depending on personal preferences easier to organise. Assuming you're not dual booting or using different file systems that is.
If it were me I'd just let Windows partition the whole drive as it sees fit and store data wherever (My Documents maybe) and use the default location for installing any games.
e: If it is to make it easier to image the drive and restore to just the partition Windows is installed to then no, having Windows installed in one partition and storage/games on a separate partition on the same drive won't effect performance and the overall capacity loss is so small (maybe 1-2MB) that it's not worth worrying about.