Fighting the temptation

I think photography is evil. I saw this Sigma 18-50mm f2.8 EX DC lens that someone in the area wanted to sell for $325. My first thought was, wow! I gotta get this lens. I have read some good reviews about this lens and a couple bad ones. This lens was praised for being fast and sharp. I was so tempted to buy it as I am planning a trip to Costa Rica. Such a lens would be very nice for the thick canopy of the rainforest. However, it took me a few hours to realize that unless I use a D700 or D3 with an f1.8 lens, there is not much I can do about the light conditions under the canopy. I will have to resort to a hotshoe flash to capture things within 13 metres from me. Besides, I already own a Nikkor 50mm f1.8 prime lens, over a full stop faster than the Sigma lens. My Nikkor 18-200mm f3.5-5.6 is only half a stop slower than the f2.8 and with VR, it should outperform the Sigma from 18-50mm, so I should after all be content with my set of glasses. I have to keep my excitement under control or I will end up spending thousands of dollars on gears that are simply redundant.