Your download speed on your PS3 is 50 kbps? First off, that is crazy. My 3G connection on my phone is incredible compared to that.
I believe you have to download the update via your PS3 console. The only updates I know of that can be downloaded on your PC and then ported to your console is the XMB update.
Your connection speed should be the same on your PS3 as it is on your PC though. "famoussasjohn" is pretty incorrect in his knowledge of PSN speeds, as they're pretty good (not quite to XBL levels though).
I'd try hooking your PS3 up directly to your router via Ethernet cable, and then in multiple ports on it. If that doesn't help, connect your PS3 directly up to the modem. If it's not getting the same connection speed as your PC, you have a curse put upon you... :O