If you can afford it, always goes for the Trifecta! It's a good investment because if you ever decide to switch to Full Frame, it'll still be useable and awesome.
However, I don't think your gf will be able to afford it. And you must face reality...
If money is an issue, and you absolutely want a lens that's a bang for your buck that's good to walk around with, I would recommend the new sigma 17-50mm. Yeah, you're thinking, I already got that range with a kit lens that's 100 bucks. Well, no you don't. The sigma is fix focal at 2.8 with Flouride element and is sharp sharp sharp and minimal ghosting and flare, with reduced CA. Look at it on DXO mark, you'll see it beating nikon 17-55mm and almost on par with the nikon 24-70mm (which is Nikon sharpest zoom. period.). All that for a lens that's 669 dollars. I think that's the best bang for the buck you can find right now. Not many reviews on it right now, cause it's relatively new, but the reviews that are out are all positive. The new F-element from Sigma is really good, AF is good, and the Optical stabilizer is good too.
I know it's not as versatile as the 18-200mm because it doesn't have the long end, but the 18-200 has notorious lens creep, non fix apature, and so so IQ. I hardly ever find myself wanting to go to 200 unless I'm an avid wild life photographer, and if i'm an avid wild life photographer, I would not be looking at the 18-200 anyway.
Anyway that's all i got to say, cheerios!
