Should I upgrade my DSLR or start investing in lenses
If your going to buy a new body and then look at lenses, I'd recommend going with a Micro 4/3rds system. The body and lenses are cheaper than Nikon DX or Canon systems. If you are set on a DSLR system with the ability to move up to a more pro body later, then I'd recommend a Nikon D5100 or D3100 or a Canon T3 or T3i. Whatever you decide, buy a 50mm 1.8 lens if you don't have one already.
50 mm (25mm) f1.4
319 for dslr
529 for micro 4/3
As for lenses, you will of course need at least one to play around. I would caution against buying a few as it mean you will spend less time learning about the effects of different focal lengths, apertures, etc.
As recommended above, one of the best lenses to have is a "thrify fifty", 50mm lens. This comes in a few flavours, but i would recommend the 50mm 1/8D (gdgt.com/nikon/af-nikkor/50mm/f18d/) as it is small, light and as sharp as hell. And best of all you should be able to pick up for around $200 or less online (in aussie dollars).
Using a "prime lens" (lens without a zoom) as above will teach you much more about photography than a zoom lens, and once you get used to usinging your feet for zoom (i.e. step back ;) you will probably have a lot of fun with the low depth of field (completely blurring out the background).
The only times you really should look at upgrading a camera body is if the mirror flipping motor actuation number gets high (500,000) because it will fail eventually, or if you have a need for the higher resolution (such as printing 11" x 14" prints or if you're doing commercial photography work). When you do upgrade your body, you'll already have high quality glass to use with it.