SLI / Crossfire old GPU with new GPU?

am starting to completely build a computer for the first time.

I have an old Radeon HD 4550 that works pretty well. I am definitely going to upgrade & get a new GPU, but I was wondering... is there a way (or is it worth it) to SLI / Crossfire my old GPU with a newer one?

I'm still fairly new to computer building, so I may have the wrong idea of SLI. It seems to me is that there are 2 cards plugged into PCIe slots and when you install certain drivers, the PC can utilize both processors' power.

Also, I'm about 90% sure I'm going to use an ASUS P8Z77-V PRO mobo with an Intel Core i5-3570K.
For my price range, the GPU's I'm looking at are: Radeon HD 7770 -or- GeForce GTX 650 TI

Thanks

 

Also, the results are random and all over the place with either.

Most games see very little benefit to multiple cards. You be better off saving the money and not buying an SLI / CF board and using the saved money on a better GPU.

As for spending in general, also make sure to buy a good quality PSU. That is one place many new builders skimp and suffer for later.