My friend bought a matrix chip (solderless mod) a few years back, and it came loose while he was playing and fried. And then it took his HDD with it. Thus I advise against solderless mods. If you can't solder, you should either get someone who can to solder for you, or you should softmod.
The neat thing about softmodding is that it can be done for free without buying anything additional for your xbox, or if you buy (or borrow/rent) 007, splinter cell, or mechassault and a memory card, and know someone with a modded xbox it can be done without even opening your xbox. I softmodded 3 xboxes yesterday, my average time was around 5 minutes each.
As for xboxes sold in a store, a rule of thumb is to check the date on the bottom and try to find the
oldest one. Its not as important for softmodding, but it gets harder to solder (or go solderless) with a higher version xbox, so in this case older is better..