Demisthene: You have 22 drives, and you're not using RAID? Did you use LVM to make the space on all of them appear to be one big partition? If so, that's not very safe...
shivaswrath: If you get a new motherboard, you'll also obviously need a new processor. Possibly not so obvious: you'll also need new ram. That old P4 will be using DDR, but a new processor will need DDR3. You may also need a new PSU (Power Supply Unit). Your graphics card should work so long as it's PCI-E. If it's AGP, then you'll need a new graphics card too. Your hard drives are probably IDE. A modern motherboard most likely won't have many IDE channels on it, as they're designed for SATA drives instead (faster). Seagate is definitely the best choice for drives. I've been working with computers for almost 25 years, and Seagate drives have proven to be the most reliable in my experience. We saw lots of WD and Samsung hdd failures at a company where I wrote software for digital video systems (we had stacks of big hdd's for the video storage). I would definitely avoid those brands. The only trustworthy brands for motherboards are MSI and Asus. Personally, I always use MSI: they're very reliable. When you add up all these factors, you may be better off getting/building a new computer altogether, and keeping your current computer as a backup machine/fileserver.
As far as processors are concerned, if any of you don't need a new computer right away, I'd hold of and get a core i9 processor instead in the first half of next year. The core i9 will have 6 cores with Hyperthreading. I'm holding off on upgrading from my dualcore for that machine myself.
If you run Mac OS X 10.6, then the GPU you have will make a difference in performance. 10.6 makes full use of OpenCL, an open standard created by Apple for distributing workload across all processing power in a system, meaning both the host CPU and the GPU on your graphics card. It's also designed to work with DSP and FPGA cards, but I don't know if the implementation is capable of supporting that yet, and I don't really want to open that can of worms anyway. OpenCL, as I said, is an open standard. Windows 7 and other OS's (such as Linux) are gaining support for it. You may not currently gain a benefit from it on an OS other than OS X 10.6, but in the future you will. If you plan to build/buy a nice system in the not too distant future which you plan to keep for more than a couple of years, it may be well worth considering getting a graphics card with a more powerful GPU (in other words, a faster video card). You may also want to opt for a motherboard which supports two video cards, so that in the future you'll be able to add a second video card if you want a performance upgrade (when utilizing OpenCL) so that you can just make that one addition rather than having to upgrade to a new system or a new motherboard/processor.
And seriously, the more ram, the better. If you're on Windows 7, Mac OS X (any version), or even Linux (also, any version), the RAM that's not being used actively by the programs you're running will be used to cache files and programs in memory so that you don't have to read them from the harddisk when you access them again. This greatly improves performance. If you're using a version of Windows older than Win7, though, you won't gain this additional benefit. This is an old feature for 99% of operating systems, but just now finally has been added to Windows in its latest incarnation.
Hope this helps.