You don't need to buy a new copy of Windows 7 to use 64 bit. You're entitled to use either 32- or 64-bit with each license. So you can use the old key or move it somewhere else if it is retail. If its Factory OEM I would def use it to not lose it's $100+ value.
Unplug the HD, plug in the SSD to SATA 1 port. Boot the Windows 7 installer to Clean Install Windows 7
doing the partitioning you want at Step 6/7 or just click Next to have Windows 7 format one large partition and proceed with install.
After install, plug back in the HD to access its file, boot it if needed using one-time BIOS Boot menu key at boot. When ready you can wipe the HD with Diskpart Clean Command
and repartition in Disk mgmt. Partition or Volume - Create New
That SSD should be large enough for OS, all Programs, the paging file set to Auto Managed, Hibernation file if you use that modern perfected feature in Windows 7, and your data. If it becomes tight copy User files or data off to HD the rightclick the copied User folder or data to link to related Library - Include a Folder - Windows 7 Forums
which is the easiest way to link data from the HDD and see it all in one view.