Powering a large drive via USB isn't going to happen, the USB port simply doesn't provide enough power.
I know you just had shoulder surgery, so this might be hard, but I would build on myself. Just buy an external enclosure, and an internal 3.5" drive and put the two together.
I like this enclosure: http://www.newegg.com/Product/Produc...82E16817332012
Mainly because of the fan to cool the drive. With an external enclosure, a fan is a must. Without one the drive will definitely overheat, and the drive's life will be cut short.
Pair it with this drive: http://www.newegg.com/Product/Produc...82E16822148261
And you have a 250GB external hard drive that has both USB and e-SATA, and the drive has a 5 year warranty, which is pretty uncommon in the external hard drive world all for right around $100.
As long as you format the drive in FAT32 you can use it with your PS3, just plug it into one of the USB ports. I use the exact same setup with my PS3, and you can move it between computers with ease.
You could use this drive if you want 320GB: http://www.newegg.com/Product/Produc...82E16822148316
With the 320GB drive the whole setup is only $125 after shipping.