Firstly, note that just because a drive has a 6GBit/sec interface, it doesn't mean that the drive is actually capable of transferring data that quickly. You don't actually need anything faster than the drive's native transfer rate, even if it has a 6Gbit/sec interface.
The motherboard you have now with it's 3Gbit/sec SATA ports will work just fine with almost all the SSDs that are currently available. Very few SSDs can transfer data faster than 3Gbit/sec, and even the few that do won't perform particularly slower because their biggest benefit is in very fast access times which aren't affected by the connection speed.
The 4X PCIe USB 3.0 / SATA 6Gbit/sec card you're referring to should also work just fine. Based on the card's specs, if you DO find a drive that can transfer data faster than 3Gbit/sec, then it should work fine with that card without being bottlenecked.