HX is the professional series, and they tend to have better components, better regulation, lower noise levels, and modular cables.
TX is more of a kind of enthusiast-on-a-budget series. It's well within ATX spec, still has high-grade japanese caps and a 5 year warranty and whatnot, with good noise levels, but it's got fixed cables and is a bit cheaper usually.
The release order went something like this:
HX520 / HX620 - Launched first (2006)
VX450 / VX550 / TX650 / TX750 - Launched next, about a year later (2007)
HX1000 - Launched a bit after that (2008)
TX850 - Launched December 2008
HX750 / HX850 - Launched last month - best PSUs in the world, in my opinion.
Of course I'm biased.