Not sure if this still applies, but last I heard DSL has lower ping time than Cable, which decreases lag (depending on what you mean by lag). For example, with cable you may notice other characters slightly "leaping" from place to place, whereas DSL has higher refresh thus less jumpiness. Again, this is historical knowledge, so if it's wrong now don't flame me.
As for sharing broadband, I have used both Cable and DSL and found both were fully sharable using my little D-Link router (or as they say, Ethernet hub with 4-port switch). As a matter of fact, not only have I done that at different times with my own network, but currently at one location I am on a network that uses cable internet, and at another a network that uses DSL. I had heard that some cable internet providers were trying to block this, as they want you to purchase an account for each computer, but I have never seen it in practice. I can't imagine how they would be able to block you if you used a router.
I agree with the suggestion that you buy more memory, especially if you're at the low end of what your system could support. Say for example you have 256MB of RAM and could take up to 1GB. Well, if you go all the way you'll see double-digit increases (percentagewise) in speed on load times for applications, which is essentially what zone loading is. You can get lots of memory for a not unreasonable amount of money. Frakkor and Grimleaper mention a couple of video cards. I don't know what it cost them, but those retail at around $350, which sounds like a bit much for your budget. You can get cheaper than that, but without your system specs I don't know what would be an upgrade for you.
Edited, Thu Nov 10 12:09:05 2005 by Nathanius