First off, never trust benchmarks and performance charts from a product's manufacturer. They'll distort them to make themselves look better.
The performance isn't quite going to be double, but you will notice huge boosts in rendering and such. Also, having two processors will enable you to do a lot of multitasking; for example, burning a DVD and rendering a video at the same time, with no slow down.
Can anyone tell me what sort of rendering bonuses I would get from using a dual processor over a single of the same type and size, or any other bonuses, or even drawbacks
I realise that the software has to be specifically wrote to use dual processors (as Premire is) is this the same for dual core processors?
Yes. Intel even has a dual-core processor with Hyper Threading, giving you 4 effective processors in a single chip.
and finally can two dual core processors be put in a dual processor motherboard and if so would it be of great advantage over either a single dual core processor or standard dual processors.
AMD Sempron 2600+ (Socket 754; 1.6GHz)
768MB Crucial PC2100 (I know, I really need PC3200 :()
ATI Radeon 9200