In all honesty, a 256GB SSD is more than enough to:
1. Install Win7 & all the updates
2. Have a good selection of normal Applications (MS Office, development tools, video stuff, etc.)
3. Hold GC3 plus a couple more games (say your 3 most common).
ALL your data files belong somewhere else, as do you less-commonly used things.
Also, your Swap file belongs on HD, NOT ON SSD. Don't put it on SSD, it will perform no better than HD and wear the SSD out prematurely.
I have a 512GB SSD installed right now, and there's a total of 100G on it:
- Win7 Pro
- MS Office (Word, Excel, Outlook, Visio)
- 3D Modelling software, Audio recording software
- 2 versions of Netbeans and 4 different JDKs
- A boatload of utilities.
- World of Warships (30G all by itself)
- GC3 and Civ3
So, yes, a 200-250G SSD will be very comfortable for you to put pretty much everything you really need on it. Just remember to periodically clean Windows of update "backups", as they can wrack up totally useless space.
I've got 200G on another HD, that holds 6 other games (3 versions of Homeworld + Mods, Sins of a Solar Empire, TotalAnnihilation +Mods, Starcraft, Civ3 Mods), my Cygwin and VirtualBox stuff, my git repos, and misc video files.
DON'T go for the PCI-E SSDs. They require modern BIOS to boot, and I doubt you have that with what sounds like a system that's at least 3+ years old.
Honestly, in your situation, pull one of the 3.5" drives and buy a USB External enclosure for it. Use that drive for backups. Put the SSD + adapter bracket in the place of the 3.5" drive you yanked.
And, yes, given your current configuration, starting from scratch as far as the OS install is probably the Right Thing at this point. It's unlikely that you'll be able to properly clone the Win7 partition if the Vista one isn't there, and just keeping around Vista is a waste of space.