I think it has to be 30GB (maybe down to 25GB) or it won't install. Once configured Windows 7 can be as small as 15GB. I know because I've had it that low (only to quickly fill up the space with programs). You have to disable the pagefile, hibernation (off by default on a desktop), and system restore.
I'd only get a 30GB if the system drive is only going to be used for the OS and a few tiny programs. I would get at least a 60/64GB if you plan to do more with the system disk, and 120/128GB if you can afford it. I got a 90GB for my PC and I offloaded the pagefile and indexing to my HDD. I'll probably move more stuff later so I can get the performance boost for things that will actually benefit from it.
I believe the 32-bit uses less space and you have less RAM so you have a smaller page file.
But if you have more ram, you need less of a page file.
One thing that can boost the ssd space is hibernation. The hiberfile needs to match the ram size. Unfortunately, once there, the hiberfile stays there, even if you disable hibernation. One needs to research ths to find out how to eliminate the hibernation file altogether.
I am not much in favor of micromanaging the os.
To my mind, I want the page file and index on the ssd for fast access.
A demand page fault stops the cpu dead until it is resolved, so I want that to happen quickly.
In normal desktop usage, the ssd will be long obsolete before it's quota of writes is depleted.
It is not worthwhile to try to minimize writes either.
I know you don't have to minimize writes, but I do need to maximize the space since I couldn't afford a huge SSD. A 90GB SSD will be filled long before I can justify buying another
Windows is stupid sometimes and with my 8GB of RAM it creates a 8GB page file by default. I don't need to waste 8GB of SSD space. I could have disabled it, but I want the page file and need the space on the SSD. The page file shouldn't really be used since active programs should be loaded completely in RAM. If I had 12-16GB of RAM I'd probably completely disable the paging file.
Same goes for indexing. I could have turned it off, but I'd rather keep the indexing feature and it is fast enough off of a HDD. While it doesn't take a lot of space (0.25GB on my old PC) it was easy enough to transfer to the HDD. I'd rather have my games and programs on the SSD then the indexing.
The other things I'm considering moving are error logs, temp files, and maybe a few other things.