i mean, it's technically capable of communicating at 8mbps, but that CPU's not going to be able to crunch encryption/decode binaries/decode, delete and move temp. files fast enough to keep out of it's own way.
Unless the 1MBps line is the SLOWEST you can get, then i wouldn't buy a 1MB line just for use with that computer. Back when people used computers like that, a 1MB t1 was meant for an entire business/apartment, not an individual computer. It wont be able to keep up with the data coming in. It'd be a waste of money.
Even my p4 3.6 (which i realize is by no means a new processor) gets choked up sometimes with my 5mbps line on big decompression jobs, and ties up the entire system, and you're talking about a connection that's almost 2x as fast.