ESXi is a standalone OS, and would not require Windows 2003. If you are going to use the Host OS Windows 2003, you can use VMware Server 2, which operates like ESXi, but has many differences. If you want to run ESXi I would head over to the Hardware Compatibility List and see if what you have is a supported setup (I don't think it is, but it is worth a check). I am pretty sure ESXi Installation will be out of the question on the hardware you listed. If that is the case, I would recommend running VMware Server 2.

ESX3.5 will install on pretty much any PII or later CPU. IDE drives are not supported, but since it does work with IDE CD drives, some have reported they have been able to get the boot partition on an IDE drive. But it's not supported so in general, you need a SCSI or SATA controller. The Promise SATA controller is likely software RAID which is not compatible with ESX. While it may work, you probably won't get RAID.

The other piece that's picky is NIC. The common Realtek NICs mostly do not work. Check the ESX compatibility list or vm-help.

Me personally, I installed ESXi3.5 on an old eMachine, PIII Celeron, 2GB RAM (Max the MB could take.) I added a $30 PCI SATA card (Si3512 chip) and some Intel 100 Pro NICs. (Onboard Realtek NIC didn't work.) Other than the SATA card, everything else was "throwaway parts." If I changed it to boot from USB stick and used a SAN for storage, I wouldn't even need the SATA card and drive.

Obviously, I'm not going to run any large VMs or use it for production, but it was a good machine to learn ESX.