Many are looking at high end cards for that, but I'd suggest a midrange card from one or two generations later.
So instead of a 9800 GTX, take a GTX 260, instead of a HD 4870, take a HD 5770, instead of a GTX 285, take a GTX 460, instead of a GTX 580, take a GTX 670 etc.
The newer midrange card will draw less power, run cooler, tend to be built smaller (since it needs less cooling), and have less time run at the limit, so it will be in better condition.
Highend cards are also more sought after, so they will take a bit of a premium. Just like with CPUs. A C2Q Q9650 or Phenom 1100T is still expensive, even today.
And let's not forget dual boot. Something like a Pentium 3 or Athlon with a Ti 4200 can run 98 SE and XP, something like a Q6600 with a 8800 GT can run XP and 7 and so on.
So I agree that there is little need for a dedicated XP system, when you set up your fleet with multi boot in mind. XP bridges a huge area of hardware, the lower end also doing fine on 9x and the higher end on later releases.
Like when going for a "latest and greatest" system with an i5-2500K and a GTX 780 Ti, which can run everything under XP maxed out will also do fine in DX 11 titles under Win 7 or even 10.
I'd argue that a late 9x/early XP system and a late XP/early 7 system would be a much better (and cheaper) choice. Unless of course a specific, period correct setup is wanted.
But in the end there is no wrong setup. If the GPU is "too strong", you can always put that performance into more eye candy. And not every game has the same requirements, some are heavier on the CPU, some on the GPU, so going with something stronger doesn't hurt, if it means having enough performance for both.
P3 933EB @1035 (7x148) | CUSL2-C | GF3Ti200 | 256M PC133cl3 @148cl3 | 98SE & XP Pro SP3
X5460 @4.1 (9x456) | P35-DS3R | GTX660Ti | 8G DDR2-800cl5 @912cl6 | XP Pro SP3 & 7 SP1
3570K @4.4 GHz | Z77-D3H | GTX1060 | 16G DDR3-1600cl9 @2133cl12 | 7 SP1