NX is what you want. Effectively, it's x11 forwarding, but with proxies on both sides that do compression and caching to remove most round-trips. It vastly accelerates x11 to the point where it's faster than pretty much anything else.
I had a friend over the other day. We were both sitting on my couch with our laptops. He had some NX windows to his home computer on a different ISP (figure ~40-50ms latency since I have a 1000km round trip to my first hop), and I was using RDP (Remote Desktop) to my desktop over my wifi.
His NX was faster than my RDP despite the fact that he was going over the net and I was local.
You can get more information here:
http://en.wikipedia.org/wiki/NX_technology
I'd suggest using the official binaries. The core of NX is GPL'd, and there are some free implementations like FreeNX, but you'll have a much easier time with the official binaries.