You have a BIOS Limit on the upper Size that you can fit to this computer. According to the Dell Service Manual it originally shipped with a 40 GIG HDD and the step up to 250 GIG or bigger is just too much for this M'Board to handle.
However if you where to fit a PCI SATA Card to this system in one of the 3 PCI Slots if they are not filled up you can then fit any size drive that the Controller Card can support and that should be all of the current ones that are available.
As this system is supposed to have a Celeron CPU it would be a idea to upgrade this component and the RAN as well as fitting a PCI Video Card to do away with the on Board Video which uses the System Memory and CPU to generate Video. A Dedicated Video Processing Unit and Video RAM are always faster than using the CPU and System RAM to generate Video and will give you a better result.
You can max out this system with 2 GIG of RAM that is 2 X ! GIG Ram Sticks which are DDR RAM and a succeeded type. These are easily replaceable and freely available and would be the single most positive thing you could do to improve the speed of this system. Then fitting a Dedicated Video Card would remove the 64 MEG of RAM that the Video steals from the system and allow the Intel 865 Chip Set to run in Dual Chanel Mode which is considerably faster than Single Chanel Mode.
If you want to replace the CPU you can fit a 3 GIG P4 which is a socket 475 CPU Type from Intel and these should be available on places like E-Bay at a very reasonable price. I have yet to have a CPU Fail unless it has been overclocked or run without a Heatsink. These P$ CPU are otherwise piratically indestructible and just work smoothly without problems. Though they do get hot and need to be properly fitted tot he Heat Sink Provided by Intel.
OH if you want to look at the service Manual it's available here
http://tinyurl.com/5wwldv
I hope that is of some assistance to you.
Col