Hey, welcome to the forums!
What I would do is look seriously at a pair of Athlons. First of all they will perform much better than a pair of 550mhz PIIIs. This has been pointed out and is obvious, but secondly it might be cheaper in the long run. A dual PIII slot 1 board and another PIII 550 CPU could cost as much as a pair of MPs and a dual socket A motherboard. Even if you saved some money with the PIIIs it would not be much and definitely not enough to justify the lackluster performance of the Pentium 3s in comparison to the new Athlons. Consider both options obviously, but think hard about going AMD for cheap performance. Then again, if your pops needs OEM-ish stability from his system for business, it might be better to go with the PIIIs because their more established technology may ultimately prove to be more stable.