Yeah, the GMAT scoring algorithm is pretty strange. You can miss tons of questions on quant and still get a 49, since your score is mostly dependent on
which questions you miss, not necessarily on how many you miss. The GMATPrep runs the same scoring algorithm as the real thing, and you'll see some crazy stuff come out of those tests--one of my students missed seven questions and still got a 51Q, another missed 20 questions and got a 44Q. As long as you nail the easier ones (whatever "easier" may mean for you), you can miss tons of hard questions and still do really well.
On verbal, the algorithm works similarly in theory, but if you're scoring a 40 or above, there seems to be a tighter relationship between your score and the number of questions missed. A 46 or above on verbal means that you only missed one or two questions. (Full disclosure: I have the completely unfair advantage of working as a GMAT tutor for a really long time, so my score isn't anywhere near as impressive as jumsumtak's.

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And yes, running through the GMATPrep problem sets (the ones that are compiled from GMATPrep test questions) will definitely inflate your GMATPrep scores. Plenty of people disagree with me on this, but I think it's best to avoid those sets until you're certain that you'll never take a GMATPrep test ever again. Feel free to use the GMATPrep Question Pack (the official one sold by GMAC) if you haven't already, though.
If you can get a 49Q/40V, you'll ultimately be fine on IR. I agree that it's wise to go through some of the practice IR stuff, but don't herniate too many brain cells on that section. You've probably already thought about this, but be sure to be as lazy as you reasonably can on the AWA and IR during the actual exam, and make sure that you eat (and drink caffeine, if that's your thing) during your breaks. Low blood sugar (or a caffeine crash) can really hurt you toward the end of the test. I doubt that it's actually an issue for you, but I figured that it wouldn't hurt to mention it, just in case.
Keep us posted, bluemints!