The difference between [the P4] and the [Athlon] die size is frigging huge - AMD's Jerry Sanders III
Then there's that review, probably the first of its kind of the Via DualGFX platform which should offer affordable SLI for both AMD and Intel platform. Two nVidia cards have actually been mounted on the boards and I do not know if it will be compatible with ATI ones. Two motherboards have been reviewed, one based on the K8T890 and the other on the PT894Pro. Both compared to their respective competitors. Benchmarks are available but bear in mind that they are at a very early stage and it shows. The last review is that of the AMD 64 x2 4800+. In this test, it gets compared only to the A64 4000+ rather than to an Intel part. You'll actually need a good chinese translator to be able to see through all the information available.
Bittech has a four part article found here, here, here and here where they explore the intricacies of nVidia's SLI platform. You don't have to guess where their hearts are as they test four SLI AMD motherboards, then 6800U and 6800GT as well as 6600GT and 6800 graphics cards and lastly a FX55 processor clocked at 2.6GHz then to 1.8Ghz and to 2.2Ghz. They discuss over the relative advantages of the four SLI variants proposed as compared to similarly priced mono GPU configuration. Their advise, buy a A64 3000+, an SLI motherboard and a 6800GT. Later on in six months, get yourself another 6800GT. Then in one year, buy a cheap dual-core processor. µ