Hmm, this actually works just fine for me with only NX client. What advantage does Xpra bring to the table?

Or just Xpra, for that matter--- Xpra was written as a replacement for NX (http://lwn.net/Articles/343389/), and they have heavily overlapping functionality, so it seems odd to use both rather than picking one.

Does Xpra offer the same level of compression as NX?

NX is designed to work fast over low bandwidth connections.

It doesn't look like it. If you need a remote desktop, NX is the way to go, in my opinion. However, I do use Xpra for long-running programs because I don't want to have a full-blown X server running all the time.