It might be a lot more cost effective and time productive if you were to get the slowest of all the latest technology; thus giving your sister the option to upgrade later.
As a result it will probably cost more but you should at least try and get a motherboard that supports AMD Athlon 64-bit. But other than that bit of nagging it looks fine.
Maybe you should consider a Intel Pentium 4
processor instead of AMD (there are too many classes in AMD at the moment). If you were to get a Intel Celeron 2.40Ghz (NORTHWOOD/128KB/400 FSB/0.13u/S478) CPU you could upgrade to a Intel Pentium 4 3.40Ghz (NORTHWOOD/512KB/800 FSB/0.13u/S478/HT) later... providing you bought a motherboard that supports both.
With the Athlon XP you listed above you are really limiting yourself for upgrades later. You should definitely re-think that motherboard also.
You should be getting FSB400 and DDR400 as standard these days, this gives you lots of upgrade options later.
Note: Kangaroo thinks motherboards are the heart of all machines
This motherboard might be a lot better, the MSI PT8 Neo-LSR.