Short answer is an Ivy-Bridge i5, GTX960 and an X-Fi is going to make for a pretty-much-as-good-as-it-gets overkill XP machine, and a very good Win7 experience, with everything just working out of the box.
Longer answer is that Ivy-Bridge with the Z77 chipset (and Ivy-Bridge-E with X79) is the last fully supported, everything is meant to work and just does platform for XP. Haswell/Z87 is partially supported (and is pretty easy to get working), and platforms beyond will work too, with increasing effort on your part, but Ivy-Bridge is plenty overkill, plenty cheap and having everything just work as intended makes life much easier all round. If you are willing to pay a premium, X79 and Ivy-Bridge-E will edge out anything Z77 (HEDT levels of cache and quad channel memory give it a small, but nice, bump), but that comes at a cost dollars wise. The X58 platform is also a good contender, but still comes in more expensive than Z77 and I don’t think you notice any real advantage from it.
For RAM, 32-bit XP isn’t going to care how much you have as long as it is at least 4GB. Win7 will love you for 16GB (or more if you are keen).
GPU wise, GTX 960 is the latest fully supported GPU for XP. The GTX 970 and 980 are fully supported too, but only with the 344.11 drivers. I’ve never had an issue with a GTX 980 on XP using these drivers, but if you want to use the most recent Nvidia drivers the 960 is the easiest way to go. Beyond that, all of Maxwell up to and including the GTX 980ti and original Titan X will work with a simple driver inf mod. The 980ti tend to have better coolers and clock better in stock configuration than the Titan X, so will likely be the best you can get for any XP overkill machine - technically it will be beaten by a Titan X with more exotic cooling and an overclock, but only you can know if you want to try to hunt down a compatible water block and build a custom water cooling loop. A 980ti is pretty beefy for Win7, but there are Win7 drivers for everything up to the RTX 3090. If the 980ti will be overkill enough for 7 will come down to how far you push your Win7 experience.
If for some reason you are keen on a Titan X, just make sure you get the Maxwell Titan X. There was another Titan X but it is Pascal (think 1080ti), and there are no XP drivers for it. Thanks to nvidia for naming two cards Titan X despite them being completely different.
If you are a ATI/AMD person, the best you can get for XP is an R9 280x (note the R9 290/290x won’t work in XP!). Performance wise that’s about a GTX 580 to 680 equivalent, well short of a 980ti. A GTX 960 probably isn’t much ahead performance wise, but power consumption and heat will be a lot lower.
An X-Fi for XP is a no brainer. That gives you support for everything EAX, and hardware accelerated audio including EAX is a big reason to play XP era games on XP.