First 2 answers partially true.
Such adapter isn't proper solution for various reasons :
- possibly not enough power supply from the slot,
- performance would be drastically lowered due to latency and slot throughput limitation
- for CF or SLI you need support from the chipset also... so you wont boost your gaming performance this way
Btw as one of above answer suggest you need mobo with extra x16 slot, is partially true... I've seen mobos with another slot but without SLI/CF support... so the correct answer would be you need a mobo with such support AND with additional x16 slot.
Another thing is that to boost performance in SLI/CF you need in most cases the same model of the GPU you already have... and most likelly you need such...