Equipment Advice?

So I'm in need of some advice for upgrading my current equipment.

Just now I have a basic Nikon D3000 with the 18-55mm kit lens (non VR) and a Tamron 70-300mm lens which has so far suited me fine as a beginner. However I'm noticing a lot of my pictures aren't particularly sharp when zoomed in and the camera struggles in low lighting situations so I need some advice on where to go.

I'm an architecture student so I need to use my camera for a wide variety of purposes, from taking records on a site visit to photographing scale models .These can be very small or quite large and the photos need to be of high enough quality to later photoshop them onto the photos of the site. These are also often taken in low light conditions with a point light source to simulate the sun which results in areas of relatively bright light and harsh shadows. I also use my camera for personal photography if I see something that interests me, such as landscapes, street photography and photos of my dog and family.

What I want to know is whether I should upgrade the camera or the lens first. I've been looking at cameras for a while and have done a lot of research and decided on the D5100 as it appears to be the best option for what I need, however with the introduction of the D7100 I thought that the price might drop on the D7000 which could make it a viable option for me so I would appreciate some input on that too. Of course I'd be happy to consider other cameras if they might be better but I do have Nikon lenses and a Nikon flashgun so I don't really want to start on a new brand. I really don't know much about lenses so I'm open to suggestions on that front. I did look briefly at ultra wide angle lenses to help with site visits and landscape photography. My ultimate goal is a PC lens but if I got one of those I'd probably have to give up food for the forseeable future.

So to summarise: I'll be upgrading the camera at some point and I want to buy some new lenses, I'm just not sure which to buy first to have the biggest impact on the quality of the photos I take.

Please keep in mind that as a student I'm on a relatively tight budget, and as an architecture student that budget's even tighter (material costs for models etc) although I do understand you get what you pay for and I will happily wait longer if it will mean I get a better quality lens/camera.

Thanks for your time.