i have a canon 300d digital slr even with the standars 18-55mm that comes with it you will be able to take the kinda pictures you want. a macro lens is a nice thing to have around. for mantids a 50mm macro would work. but if you wanna get top gun get a sigma 105mm exdg macro.
you might find a 300d on ebay for around 700 bucks now . when i got it, it was like 1100 but the new 350d thats out is a step better and is only like 900 bucks
oh and when looking into cameras dotn get fooled by megapixles. if you dont have lens quality all the megapixles in the world wont help you. so if your serious about shooting get a SLR wether you get canon or nikon (the 2 best)
all of these were shot with my 300d www.pbase.com/hortus
if you get a canon you can learn everything about it here
http://www.digitalrebelforum.com/phpBB2/index.php like the mantid forum theres alot of great people
oh and if you want to break into doing photos of other stuff theres over 300 diffrent lenses to choose from. trust me you will defiantly want a macro, 50mm portrait, standard zoom 18-55. a wide angle good for alot fo stuff and probably a telephoto (zooms are more handy but primes have better quality)