Simple answer: check benchmarks to see if the XL is faster than the Pro in proportion to its 20% higher price. If so, then you may consider it, and just buy whichever one offers you the framerates you want at the resolution you want in the games you play. If it's a worse buy, and you're watching your money, why not just get the Pro? It's not exactly a slouch, and the less you spend on aging tech like the X800 series, the sooner you'll be able to justify upgrading to a second-generation SM3 card.
If you mainly play CS:S, that makes it easy: just look for CS:S benchmarks. I think TechReport should have some in their XL review. Just make sure the PCIe XL and the AGP Pro in the benchmarks are using the same processor. Some sites use a P4 for PCIe and an A64 for AGP, so the PCIe cards are at a disadvantage right off the bat b/c of the P4's relatively slower games performance.