As I expected/feared, Cablevision still blocks port 80 so I had to move my site to external hosting. I signed up with JustHost.com, which has (supposedly) unlimited storage and bandwidth transfer for ~$5/month. Not happy about their DNS handling, and that my 404 pages are apparently filled with ads & popups now, but with the discount code I found it came to about $42 for a year of hosting. Not bad.
Still cleaning up, hopefully all the existing links will work. WordPress’s import function is pretty good but I wasn’t able to pull the attachments off the old server (due to the port 80 block).
At least this seems way faster than my old Athlon box.
Edit: Ping times have gotten much better since the move: