And not all SSD are created equal. I just bought my second, a OCZ Vertex 4, and it SMOKES the OWC junk I bought 3 months ago. Avoid OWC SSD's for Photoshop. Get the fastest SSD designed to work with uncompressed date you can afford, even if your bus speed is only 3GB/sec. And like all drives, they usually perform MUCH better if kept below 50% full. I am currently using a 128GB SSD and it is fully loaded with all my programs including PS CS6 and it is <20GB full, and that includes a 2.67GB .tif and a 3.32GB .psb that I'm retouching and will move back to my RAID when I'm done.
As far as Drive Space is concerned a key factor, as I understand it, is getting a drive sufficiently large to handle all your programs and keep a suitable workspace for PS files without going over 50% full, that way the drive always performs at it's fastest. One step which aids this is moving your Home folder (which accumulates HUGE amounts of data) to another drive, like your image storage RAID (which is what I've done) or wherever you have sufficient room on a fast bus. With a laptop drive bays are more limited and space is at a premium. But the optical bay can be converted into an SSD bay with an adapter, so you could easily put in two SSD's total, one for programs and workspace and one for your PS scratch disc, or break it up anyway that makes sense for you, and even have an outside RAID array for mass storage.
And MAX OUT your RAM.
I like Mac's because they are very easy to understand, clear and uncomplicated file names and filing system and really easy to wipe and reinstall and keep clean and maximized, but OSX 10.8 does not support anything earlier than 2008 or 2009 i think, so that may be a consideration if you get a used machine. A used machine with an i5 processor and max ram and a couple SSD's with a RAID that stays at home could be a smokin little system.
I am not a computer guy, but these are some of the things I've learned recently by researching my own upgrades. I hope it helps.
edit: And, I just remembered, 2 SSD's set up as a RAID 0 provide an even greater boost to speed and efficiency. So, that may be something to look into for your laptop. Get two 256GB SSD's, like two Vertex 4's or whatever, and set them up as RAID 0 and store your masses of images on a separate RAID at home and keep a smaller portable HDD for transfers, storing music and movies and running around town or whatever. I'm really focused on maximizing PS editing and not maximizing a lifestyle facebook machine.
Also, SSD's don't have moving parts so they run MUCH cooler and quieter than HDD's.
And on a Mac it's really easy to try different configurations to see what works best for you. Just set up your drive normally with all your email settings and passwords and browsers links and PS settings etc., then duplicate or copy the drive to an independent drive or whatever so you have a full clean copy to move around and experiment with. Then try a few different configurations to see what you like, and you can still keep the copy to reinstall at a later date like in a year or so just for a quick and easy fresh install. Then update the software and copy that for next time, easy peasy.