Windows 7: USB3 drive for swap, registry and temp files ?

Moving temp files, cache, similar out of the SSD is actually a good idea, not for read-write cycles (as that number is so huge that you don't need to care about it unless you are running a server farm with hundreds of GB moved per day), but for sheer performance. Your choice of USB3 is poor. It will hurt performance.
Screw USB3, and make a RAMdisk out of some of your RAM and use that for browser cache and temporary internet files, system's temp files and whatnot. Being RAM put to work in another way, it will smoke even SSD performance. An article with more details.

About pagefile, since you have 8 gigs of ram you don't really need a swapfile (that's disk memory used for emergency situations when ram is low, if you need more ram and you are at 100% the less-used data is dumped from RAM to that dedicated disk space), as it's pretty damn hard to fill 8 gigs unless you like to keep 30+ browser tabs open. Nowadays it is rarely if ever used for its real purpose, but since it is needed for some minor error logging you can easily reduce it to 200 mb with no impact. (yes, you could place it in a ramdisk but it would be kinda pointless as i explained above and you need error logs to last after system restart, while a ramdisk is erased when the computer is shut down just like normal RAM).

I think moving registry is completely pointless, if possible at all. As the name implies, that's a list of all stuff for the OS to lookup whenever it needs it, it's not anywhere near write-intensive (unless you are an obsessive registry cleaner user or you install/uninstall tons of things per day), so leaving it in the SSD is the best (and only) choice.

I do not think I use ALL the memory I have now...

meh, readyboost is basically irrelevant when the machine has more than 3 GB of RAM, and even on my atom 455 netbook with 1 GB of ram it's hard to notice (with 2x USB 2 and a SD card anyway).

actually, if you check with performance monitor you'll see that a lot if not all ram is filled but "available" (there will be little "free" memory). That's a Windows 7 feature called SuperPrefetch, (a service running all the time) that tries to track your program usage behavior and preloads most used programs in RAM so that when you need them they load faster (as they are already there duh!).
In vista it was yet another performance hog, but in Windows 7 it rocks. If something needs more ram the preloaded stuff gives way as it should (in vista this wasn't so straightforward so you got an endless battle for RAM).
Most users take it for granted as it is always on by default, but try disabling it on a HDD and you will pull your hairs off.

yeah, I heard people saying exFAT was better because you could format them to use cluster size of 32 MB so it would be "faster" because it read 32 MB at once with less overhead. I left them exFAT because otherwise BIOS would keep trying to boot from them even if I told him not to. Also I heard that if you plug in more than one (not in ports that share the same header mind me) again that's better as it goes RAID-like.

I frankly never saw so huge differences on either this machine (or similar machines) or the atom netbook I talked about above. But I never tried for more than a few days. Hmmm.... I have a bin of 4 GB usb2 drives none will ever use anymore, might as well try leaving a couple of them in for longer.
Are your USB sticks 2.0 or 3.0?

meh, readyboost is basically irrelevant when the machine has more than 3 GB of RAM, and even on my atom 455 netbook with 1 GB of ram it's hard to notice (with 2x USB 2 and a SD card anyway).

actually, if you check with performance monitor you'll see that a lot if not all ram is filled but "available" (there will be little "free" memory). That's a Windows 7 feature called SuperPrefetch, (a service running all the time) that tries to track your program usage behavior and preloads most used programs in RAM so that when you need them they load faster (as they are already there duh!).
In vista it was yet another performance hog, but in Windows 7 it rocks. If something needs more ram the preloaded stuff gives way as it should (in vista this wasn't so straightforward so you got an endless battle for RAM).
Most users take it for granted as it is always on by default, but try disabling it on a HDD and you will pull your hairs off.

I don't know if you need it (theoretically it should still make a difference as ram still laughs at SSD in both speed and seek time), but won't hurt ram usage as prefetched stuff can be dropped anytime to make space for whatever needs more ram. You can try disabling it (it's a service, go in the service tab of task manager and stop it) to see if you notice some difference. THen maybe share the experience.