i just finished building my quad machine too, and the first test i did was i mini- complicated scene in vue 5 infinate, now the quad machine rendered the scene in two minutes, using all four cores, but the P4 machine took twelve minutes using two cores, so that it quite an improvement.

also the preview quality, and amount of polygons allowed before things start disapearing from view, is able to be much higher and the end result render is kinda stunning me at the mo' as it seems to be so smooth and my own poser skin textures seem to be working perfectly, which they never looked quite right to me on the P4 machine, also the plants seem to of improved in appearance too, so it's a bit like a new toy for me at the mo'.

so i would say go for it, the price of quads is so low now as there are about to be ( if they are not already out) 8 core processors released  ( new Xeon)  and mobo's are gonna be able to house 32 gigs of ram!!