Seagate or Western Digital are fine, but most reputable make external drives will be fine. 3.5" drives are generally a bit cheaper than the 2.5" laptop ones. I didn't even know you could get USB powered hard drives (I thought they all had to be connected to the mains, shows how outta date I became since I got my PS3).
As for access speeds, there really isn't that much between 5400rpm & 7200rpm, fractions of seconds here & there.
You'll get a 250GB USB powered Freecom hard drive for around 40GBP (my pound sign on this work PC has gone kaplooey again). For that kind of money you could get up to a 500GB 3.5" mains powered HDD.You can pick up 1TB 3.5" mains powered HDD's for around the 65GBP mark.
I'd check out my favourite PC shop.
cclonline.com
They have shed loads of external hard drives from really cheap to ridiculously expensive.
Just remember you'll have to format it on the PC as FAT32 using a 3rd party program, otherwise the PS3 won't see it.
Yay, my 100th post, now I'm speshal