Hmm, I dunno. If you use programs like iPhoto a lot, which tend to read the entire contents of your itunes library at once an external USB drive could be penalizing you when compared to an internal if not only due to the slower connection, possibly negating any advantage of a SSD. You could buy a Firewire 800 drive to negate the penalty based on the traditional 2.5 drive specs I saw on the first couple pages of Google but Firewire drives will be a little more costly just to maintain the status quo, where you might be able to better balance the load by buying a slightly larger SSD than what you think is necessary to keep your most frequently accessed multimedia in to see adequately increased performance. I'm not sure but I think the iPhoto library has to be kept on the primary drive anyway, in your user's folder.
That doesn't mean I'm necessarily against the idea of a 64 gig SSD but I have no idea what drive might best achieve your needs because you've only given me the performance enhancing tactic you intend to use, not your long term storage use strategy. Applications come in a wide array of sizes, ranging anywhere from as little as 7 megabytes in the form of Dictionary up to a sizes of 8 Gigs like Valve's upcoming Portal 2, just to show you the extremes.
Assuming you do mostly casual stuff, such as myself and don't use very, I'd count on your most frequently used applications weighing in at about 500 megabytes average. Say you use twenty of these regularly, that's approximately 6 gigs of space. The O.S. itself only needs 10, which I hear can be trimmed down to 8, albeit iLife takes an extra gig of space on top of that. That leaves you with about 15 gigs of necessary space. You'll want to double these figures for purposes of future expansion... so a 32 gigs drive might be doable (A 32 gig drive is more standardized than 30). With my suggestion above though, to get a slightly larger drive for storing the iPhoto library you'll probably want the next or two up from that which is about 40-50. Then there's all the infrequently used apps, so yeah... I'd say about 60-64 would be about right, Assuming you don't need to use Bootcamp for any reason. Windows must be installed on the SATA connection link.
Also since you're on a budget are you sure a 500g external HDD won't do? My last computer had 320 internal and I was just about to fill it after two years of use for everything.
