Bash,
Hopefully you will have gained some knowledge of hardware from the link provided by yraq.
As for my advice, I refer back to my comment in #23.
I had a Pavilion 754v, which had very similar specifications to your current 750v, except it had a P4 2.4 Ghz processor on the same 400 Mhz front side bus. I unfortunately did not have an AGP slot (big mistake buying a box with a motherboard that had onboard graphic abilities only, this was very common for workstations a few years back) - but I digress.
The upgradability of that box (again, similar to yours) was limited to (1) memory and/or a 'PCI' slot card (which is okay for audio devices, and some other peripheral devices - but not video).
With your motherboard including an AGP (Accelerated Graphics Port) slot, I suggested the memory and video card upgrades as realistic performance improvements that would not - IMHO - be 'false economy' for your machine.
As for the processor, you could theoretically use the processor I linked to in comment #32, based on the possibliliy that your current heat-sink (probably a big aluminium block with fins, and perhaps a fan) is rated for up to a P4 2.4 Ghz processor. The processor I linked to is an OEM (Official Equipment Manufacturer) product, and does not include a heat-sink in the box, due to the fact that OEM products are meant for either computer manufacturers or private system builders. Newegg provides many OEM products for 'do-it-yourself' type people so that they can pass on the lower prices to consumers.
If you read the article linked by yraq, you should have a better idea on what you are getting into if you decide to upgrade the processor.
If you are of a mind to purchase (or build) a machine that is capable of running Vista and all the applications you may use (53 processes tells me you have a lot running on your current box), then it would probably be better (as you mentioned above) to simply upgrade the memory (and perhaps the video card) to allow you the use of a decent XP box while you save money for a significantly better system.
Things that would create issues for you using the current box to upgrade to a machine equivelent to a current mainstream system (as mentioned in part by others, specifically 'Starkers'):
- Power Suppy
- Motherboard
- Processor
- Memory for the new motherboard
- Video card
- Probably a new case (due to the heat issues that would arise from placing the new components in a MicroATX case, which I must assume you currently have due to the MicroATX motherboard you have).
Final note - the processor I linked to was one of two processors that Newegg had that would possibly work, the other being a P4 2.8 Ghz - which your current heat-sink may not be able to cool efficiently. There were no P4 processors (socket 478)available at the 400 Mhz bus speed, indicating that these processors are now obsolescent (if not obsolete).