Dell, what desktop badge number,motherboard, ram , PSU? My Dell Dimension 521 came with a 305 watt PSU, as long as you are not using a high powered video card, it should be good enough. I actually replaced mine with a CoolerMaster 550 watt PSU, the motherboard was 24 pin. The Dell PSU fitting is different to a regular PSU so only two screws would fit, good enough, it doesn't move.
If your motherboard is only 20 pin connector then you will need an AT PSU as they provide more power on their 3.3v and 5v rails that the older model motherboards preferred, rather than the 12v rail. You could acquire an ATX PSU as long as it had adequate watts on the 3.3v and 5v rails and use a 24 pin to20 pin cable connector adapter or the 24 pin connector may be split at 20 + 4pin.
You will need the original OS XP SP3 install disk to format the hdd but I'd advise against it ,as only a last resort.
You also have to consider the ram, whether it be mixed brands or if actually suited to the motherboard. Most Dell PC's seem to prefer 1.8v rather than 1.5v but you'd need to check manufacturer website.
What is your motherboard model and brand?