Correct. The 1519X has no AGP slot or PCI-E slot. The ES/IT have PCI-Express slots, so if you haven't ordered this PC yet, I'd strongly recommend cancelling the order and going with something with either AGP or PCI-Express slots for adding a 3rd party videocard.
If you're already locked into buying this PC, you can add a DX8.0/8.1 class PCI videocard, such as the Radeon 9200/9250. Unfortunately, these are indeed a couple generations behind and therefore will not offer the best experience for more recent games. They are good, quality 3D cards for those strapped to PCI-only systems, but any more modern PCI-E/AGP card would benefit by sizeable gains in performance.