How many 9800GT can i have (SLI)

You can only run two 9800GT's its the GTX's that can be run in Tri-SLi.

It might be possible but it is far from tested but I have thought about changing the ID with in the bios on my 9800gt to ware the driver would detect the 9800gt as some other G92 card like a GTS250 with out changing the NV mask on the shader units or changing any thing else for an asymmetric sli how ever three way might work but any gains will be fairly small due to the sli connectors which account for 5-10% of the total performance VS the pci-e only. Basically attempting to achieve a similar effect as seen in crossfirex for example a 4870 in crossfire with a 4830 or a 4850. The trick is in the bios and the gpu core must be the same as well vram preferably but the core is non negotiable. For 9800gtx owners who want sli with out shelling out loads for a over priced 9800gtx when a gts250 would be normally cheaper or those who have the 9800gtx+ and have a older 65nm version. In the end its the driver which is deliberate to keep people from buying a lower end card to sli with there existing card so they would have to purchase a like card which makes nvidia extra $$$. It would be odd to say the least to see a GTX295 in 3way sli with a GTX275 but with only two pci-e slots.