How can I upgrade a Shimano FC-TX70 crankset?
You can nominally replace the TX70 crank with any 3x7 speed, square taper axle crank with the same chainring sizes (or different sizes if you want a different gear ratio range).

However, you need to make sure the new crank maintains the same chainline as the TX70. (Chainline is the distance from the centerline of the frame to the middle chainring, 47.5-50 mm for a MTB triple.) Different model cranks require a different axle length for a given chainline, so the challenge is finding the crank and bottom bracket specifications and matching them up. It's often easier to replace both the crank and bottom bracket as you can select a pair that give the correct chainline. 

Shimano provides an extensive archive of specifications. I found the FC-TX70 in the 2004-2005 document. It says the FC-TX80 provides a 47.5mm chainline with a BB-UN25 D-NL bottom bracket. This is the only option given so we can assume your bike has a 47.5mm chainline. Finding the specifications for the BB-UN25 we see the D-NL version has an axle length of 122.5mm. The archive specification documents are very long, but you can easily use search to find the FC-TX70 and BB-UN25 product codes.

Looking at the current crank specifications and taking the FC-TY501 crank for example (again, use search to find the specs for that crank model), that crank provides 47.5mm chainline with a BB-UN100 or BB-UN26 bottom bracket, D-NL version. Looking up the BB-UN26, the D-NL version has a axle length of 122mm. 

Bingo!

You can obviously go through the same process with the other current Shimano cranks, figure out which ones will replace the TX70 directly or will require a new BB.