That is pointless, to be honest.
PCI can double the USB 2.0 High Speed throughput, but cannot compete with USB 3.0. An ideal, single-connection USB 2.0 would have about 60MB/s, whereas PCI can deliver 133MB/s. USB 3.0 can deliver 625MB/s (1.25GB/s for USB 3.1).
OP, I'd suggest forgetting the idea of getting USB 3.0 ports. They won't serve an actual function for you, because a device that can be connected with USB 3.0 can be connected to 2.0, it will just run slower. Considering only storage devices of any kind can saturate USB 2.0, this is only an issue if you need fast external storage.
If you have the desire for faster external drives, I'd suggest eSATA.
An internal SATA to eSATA adapter bracket and an eSATA-capable drive or eSATA enclosure will be exactly what you'd need for faster external disk throughput.
http://www.amazon.com/Monoprice-107...sim_pc_11?ie=UTF8&refRID=1CVX41JXM3H0MA6SDNKZ
Even the oldest iteration of SATA will outpace conventional PCI, delivering 150MB/s. SATA II has 300MB/s to offer, and SATA III can provide 600MB/s.
If you are looking for faster card reader or USB stick (aka thumb drive) support, well, USB 3.0 doesn't exactly bring that much for smaller capacity devices. An external SSD like Samsung offers? Yes, those will saturate USB 3.0, but any USB 3.0 stick or even the faster SD Cards can rarely, rarely achieve the throughput numbers they boast on the packaging. Most storage will be more randomized and end up going much slower due to the way file systems work on the small memory packages in those devices. An SSD has so many memory packages that it can produce faster rates.
You might get faster read speeds copying data off a stick or SD card, but most often, it won't be significantly faster compared to saturating USB 2.0.
If you are intent on the fastest external storage possible, go with eSATA for the time being. Be it an SSD or standard HDD in an external enclosure, you'll get the fastest useable speeds with eSATA in your situation.
That's my suggestion, especially if you are content with holding off any major system rebuilds (new motherboard and CPU) for a long while.
Do note that even the fastest SSD on SATA III isn't significantly faster in real-world use than if it is on SATA II. It certainly shows in benchmarks, but in real daily usage, the difference is THAT drastic. Enough to notice for power users in some instances, but not necessarily all the time.
Which is to say, whether you have a 300MB/s connection or 600MB/s connection, it isn't going to really be a big issue, especially if it's a standard external HDD. You would notice USB 3.0 over USB 2.0, but connected to eSATA using SATA II, that drive will not saturate the connection, and you have zero throughput difference between USB 3.0 and SATA II, heck, the drive might not saturate SATA I.