24-pin plugs will fit in a 20-pin motherboard if there's clearance on the +4 pin side of the connector. Or you can get a cheap 24-to-20 (or even 20-to-20, since again it will fit) extension.
Re: wattage, I would say ~100W is a typical load for an XP-era PC unless you're doing something extreme.
Re: new vs. old: New PSUs are really good at silence. The fan controllers are smarter than the "always on" of the early-ish 2000s, and the fans are better (quieter by design) too. I wouldn't say a whole lot has changed in switching PSU technology, so they aren't inherently better -- and due to everyone wanting shiny new components for swap-meat prices, may be worse. New stuff might be slightly more efficient due to optimized algorithms, or advancements in transistor design, but if you're not running it 24/7 that's probably not enough to register on the power bill.
The most common failure mode of older PSUs is capacitor aging and failed fans. If you're able to refurbish an older PSU by recapping and replacing the fan, then IMO, it's a wash. A good PSU is a good PSU.