Help deciding on a MoBo and Processor!!

Hi,
This is my first post on these forums. I have been lurkinig around on this site from a long time, but haven't had chance to post and here ya go,

I am thinking of replacing a Dell PWS 650 (which is having a single xeon processor and has an awful SCSI HDD speed of 15MBps which is acceptable theoritical value as per Dell support) with a new self build system.

I am looking around on various MoBo manufacturers sites and couldn't be able to decide on the actual base system that I would need to run my research applications.
To be brief, my applications contain highly PCI I/O transfer intensive applications which are multi-threaded MFC programs. We mainly run 2 application with 2 threads each most of the time and either of them are memory intensive applications. On the other hand I need a MoBo which is haveing ATLEAST 3x PCI-X slots and 2x PCI slots and also I need to have high HDD throughput (atleast 70-80MBps or more). I dont mind to spend some extra money to build a good system, but at the end I need to meet all my criteria/specs. Till now I am looking at dual processor systems, but I am not sure if I have to go only with a Dual Processor systems.

Any help/advice is greatly appreciated.

thanks,
-Pavan