I don't know any 50mm f1.8 Sigma.
Your Sigma macro lens will still be worthwhile, no reason to not use it. The 70-210 Nikon is also still a fine lens in its class.
The non autofocus 28mm Sigma may be not of much use on a D50, I think.
Do keep in mind that due to the smaller sensor compared to 35mm film the field of view is more narrow on all lenses, so to understand what field of view you get, multiply the focal lengths with 1.5.
So a 50mm becomes a 75mm lens in effect.
If you want to have a lens with about the same field of view as a 50mm lens on your old SLR, check out the 35mm f2 Nikkor, or the 30mm f1.4 Sigma. These can be considered to be the new "standard" lenses on 1.5 x crop factor cameras like the D50.
To give you a wider focal range (which you lost due to the crop factor of 1.5), you could do worse than getting the cheap but optically surprisingly good kitlens that belongs to the D50, the 18-55 kit lens. It will allow the same field of view at 18mm as your old 28mm Sigma gave.
(18 x 1.5 = 27mm)
Canon EOS 350D, Tamron SP AF 90mm F/2.8 macro, Sigma 18-50mm F2.8 DC EX, Canon EF 70-200mm f/4 L USM, Tokina AT-X124 Pro 12-24mm F4, Soligor 1.7x C/D4 DG Teleconvertor, Manfrotto 724B tripod, Canon Powershot S30