I have an E510.
But, without looking inside I can't say if the length of the card will interfere with anything. I don't think you will have problems with the dual slot (rear panel dual slot). I can't access my E510 right now to see if it will work. I have an NVIDIA 8600GT in mine, I don't know how the card length compares to the GTX260, but the 8600GT fits without problems (it was an upgrade to the original video card that came with the PC).
As you are building a new PC (what I did instead of any additonal upgrades to my "old technology" E510) and apparently going to use this card in the new PC, just try it and see if it will fit. There's nothing to lose since you are going to use the video card in the new PC.
Dimension E510 with D925 CPU and Vista Home Premium 32 bit
Home Built PC with Q6600 CPU, 8GB RAM, HD4850 Video, HTO Striker 7.1 Audio Card - Windows 7 64 bit Professional
Sonar 8.5.1 ( Producer) DAW Software/Focusrite Saffire Pro 40 Recording Interface and Frontier Tranzport wireless controller.