If you do 60"x60" size prints and want photo quality at 300 ppi, then you will need 324 MP technically for optimal noiseless print out of the camera. There is no such camera. Even the Hasselblad has only 39 MP, and the new Canon EOS 1Ds Mark III has only 21 MP. So making your 60x60 print will need some software help. There is a lot of software that can help you enlarge photo and preserve details by either interpolation or fractal methods.

So practically you just get the D-SLR you can afford and then enlarge it using these softwares.

You can consider the same brand of D-SLR as your film SLR, and save money on your lenses if they can fit on the D-SLR. You need to check with the manufacturer on lens compatibility. Most D-SLR have good performance with some minor difference, but this can become signficant when you enlarge to 60x60 print. Canon has the best low light performance when using high ISO, but this depends also on what lens you use of course. However the new Nikons are pretty close to Canon. So either one is a good choice. For large prints, you will be happier with full frame than those with smaller sensors. The full frame sensors have bigger photodiodes and thus has less noise, and will give you cleaner enlargements. The smaller the sensors, the more noise you will see in the enlargements.
The order of preference will be medium format (Hasselblad), then full frame, then APS size sensor (1.5-1.6x crop factor), then 4/3 system (2x crop factor). The most affordable full frame camera is the Canon EOS 5D but it is more than your budget. So you will likely look at the APS size cameras, with Canon and Nikon as top choice. You can probably skip the 4/3 system. There are noise reduction software like Noise Ninja etc that help to improve low light shots.
Perhaps you should borrow/rent a D-SLR and try out enlargements with a software program or send it in for professional enlargement, and see whether you think it is comparable to your film SLR. In comparison, the D-SLR tends to see movement blurring in low light action shots more than film SLR, perhaps due to higher sensitivities of the digital sensors. So you need faster shutter speeds to stop action in low light using D-SLR.