If you want to do sli with "new" cards, you are definitely going to experience bottleneck/compatibility issues with your current system. I would not recommend going this route unless you plan on upgrading your cpu/motherboard/ram/psu also... thus, building a new system.
grumpsx What is the purpose of your upgrade? For example, can you name 1-3 games that you're most interested in playing right now, and at which resolution? I'm guessing 1280 x 720. Are you only interested in running upgraded GPUs in SLI, or would you be open to a single card? What is your upgrade budget and in which currency is it?
Side note: The 8800 GTX may be 11.5 years old, but it was a beast of a card back in it's day. It also had a $599 sale price, so two of them was quite an investment back than. Also interesting is that the SATA standard wasn't finalized until 2009, therefore I presume that your storage is powered by a 4-pin molex connector, and connected to data via IDE.
If your CPU is a Core 2 Quad Q6600, then you would be money ahead to stay with a single card. The GTX 1050 Ti will tax the aging processor as it is. Anything more and you will be severely CPU bottlenecked. The single modern GTX 1050 Ti will be much faster than the dual 8800 GTX cards you have now.