I've read that there is no limit. However I can suffice with 1tb.
In all honesty, I had a 250GB 5400rpm (the original top-of-the-line slim) and now its a 320GB 7200rpm, but if I could've saved money on a 250GB version of the same faster drive, I would have, but it wasn't available, so for $84AUD delivered, the drive I chose was a good option.
There are conflicting reports about SSD vs Sata II 7200rpm, people say that the SSDs are better at reading many smaller files, but then there's the bandwith max that the 7200 seems to be at, and most of the time its a dead heat between the SSD and the 7200, meaning both are beyond the speed of the PS3, which is the buffer.
750GB really? The amount of extra space would be wasted; sure, you could always use the drive later one when the PS3 is used less, but chances are you won't considering 750GB won't be that big shortly. You currently occupy 125GB (160-35?), you will now have 625GB extra space. Do what you will, but I think a 320 is massive for the requirements of the PS3, even if you're using PlayTV or something else in a 'media centre' type way, 500GB is ginormous.
Honestly though, being an avid GT5 player, the speed upgrade is noticable, and even if the SSD is a tad faster, which I personally doubt (on the PS3, not on PC, different story), it'll cost so much more for a micron of the space (you'd pick the 256, since its more than 160, but it'll cost WAAY more than a 7200 2.5), so, let us know what you pick, but I don't see why you wouldn't select 320 or at the most 500.