For a low price you could start with the Sigma 70-300 APO Super Macro II. It has a decent macro in the 200-300mm range and sells for about $200 or less. You must use a tripod for this lens with the macro on due to the large focal range and higher aperture (5.6). It's a good mix of telephoto and macro (not a 1:1 ratio however, more a really-close-up lens than a true macro)
For true macro (1:1 reproduction) I recommend the Nikon 60mm. It is well-priced, incredibley sharp, light, and works very well handheld.
Here's an example, the drop is only 4mm in size:
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However it is a short lens for focal range so many prefer the Tamron 90mm or the Nikon 105mm.
What do you prefer to shoot? Bugs require longer focal ranges or they fly off, static objects don't require any range so the 60mm would be fine.
Do you need wider than the kit lens (do you have the kit lens?)?
For the moment there is a pretty small selection of really wide-angle lenses, so some idea of how wide you want may be helpful.
For true DSLR wide your best bet for cost would be the Tokina 12-24. It is really wide and many believe it is better than the much-more expensive Nikon 12-24 (I have the Nikon so I choose to ignore these rumours ) This range is amazing and my 12-24 lives on one of my bodies all the time. I love wide....and I love mid....heck I even like tele.............but this has nothing to do with this......
If 12-24 is not necessary there is the famed Nikon 24mm f/2.8 which rates very high but for DSLR it is not really that wide.
Honestly, start with the kit lens, if it is not wide enough the Tokina 12-24 is your best bet. It is about $450 plus.
A macro zoom compared to a macro prime is no contest when demanding a "true" macro. The zooms never reproduce a 1:1 scale so they never really get super-close like a macro-designated lens will. Plus as usual a prime tends to exceed a zoom in terms of sharpeness and distortion as the lens only need to accomadate one focal range exclusively.
For the price look at either the Nikon 60mm or the Tamron 90mm Di.
One other option to try is buy a Canon 500D close-up lens thingy. It is a dual filter element that screws onto a telephoto (usually 77mm filter) and in simple terms creates a really high quality macro. As it is just a really big filter the Canon adaptibility is no issue. On your Sigma 70-200 this would make a wicked macro with no loss of sharpness, colour, or most importantly no aperture loss (2.8 stays 2.8). The 500D is not that expensive and means you don't need to carry a second lens. The drawback is a necessity for a tripod as 200mm requires a solid base.
Look the options up on the net and decide which will suit your needs best.
For the wide...go Tokina 12-24. For the cost and being near $400 you can't lose.
Personally I have and love the Tamron 90 f2.8. It is a great lens. I picked mine up used for about $200 a few years ago (I know I got a great deal on it). However, if I had to replace it today I would buy another Tamron in a heart beat.
As far as focal length, I consider the 90-105 range the best all around length. I find it is a little short when trying to shot insects, but most other subjects are great. This length lens has the added benefit of being a great portrait lens.
If you can afford it, the Micro-Nikkor 70-180 f/4.5-5.6 combines a nice zooming capability with outstanding optics. The zoom comes in really handy when you frame the subject and need just a wee bit more (or less) within the frame. This lens has a close-up performance virtually matching the other Micro-Nikkors, and in some respects it is unrivalled (for example, the design doesn't lose light when it is focused closer unlike all other lenses).
I'm at the same place as Joel. If you can afford a prime macro lens go for the Nikon micro 60mm or the Tamron macro 90mm. IMHO they are equivalent in sharpness, contrast and color neutrality. The Tamron has a bit longer working distance that is handy with insects. Otherwise, try out the 500D.
The kit lens might be all you need. Unless you feel like you need to go under the 20mm field of view equivalent, then the 12-24 is magnificent. The Nikon version is excellent. Read the others' comments about the Sigma. I too have heard good things about the Tokina.
If you're going that wide (12 or 18 equiv) learn about shooting with that wide an angle. Once you get it it's so much fun.