Would you be buying the PNY from a store where it would be easily returnable? If so and you have verified returnability, go ahead and try it. Put it in, see if it boots your OS. So far, so good. Then run extended cycles of Memtest86. So far, so good. Run everything you might ever throw at it for about a week. If all goes well, nothing different than before except the added performance from the additional memory, keep it. If not, take it back. Simple... Of course, don't do anything dumb like cutting out the UPC label for a rebate until you're satisfied with it.
.bh.
p.s. Office Max has a good deal (after MIR) on Kingston 256MB DDRthis week. I would go with that before PNY.bh.