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What focal length eyepiece do you have already? I'd guess it is probably around 25mm. Magnification = focal length of scope / focal length of eyepiece. Ideal magnification for any scope is roughly the same as the diameter of the objective measured in mm - the "sweet spot" (i.e. roughly 125x for your ETX), although of course you can go higher. I'm not sure what the focal length of the ETX125 is, but you should choose your eyepieces (and more importantly, your targets) accordingly.
The second eyepiece suggestion is usually somewhere around 10mm. A 2x barlow doubles the magnification of any eyepiece you use with it. With your 25mm it would give you equivalent of a 12.5mm focal length eyepiece, and with a 10mm the equivalent of a 5mm (which is pushing it, IMO, for your scope). Beware that eyepieces that are physically short (even such as a 10mm plossl) might get buried between the ETX's fork arms when you are looking at objects that are near zenith. This is an advantage of the barlow. (You can buy extension tubes for eyepieces, so the problem is not insuperable. Buy a Flexifocus to replace that tiny focusing knob while you're at it.)
All the best,
Brian.
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