I meant to give you a few examples when I wrote my post this morning, but didn't have time - had to run out to do some errands. So, now that I'm back, I'd like to to add a little bit of info/details, take a look at these cards as a few examples:
Typical USB expansion card:
https://www.amazon.com/SEDNA-Express-Ad ... rd+renesas
The above card has 4 ports, but a SINGLE chip to handle traffic from all 4 ports. Therefore, you get 5Gbps of bandwidth SHARED between all 4 ports.
Here's another card, also Renesas chipset, but it has FOUR processors for FOUR ports:
https://www.amazon.com/Express-SuperSpe ... b_title_ce
This card has FOUR SEPARATE USB3 processors, each of which can handle up to 5Gbps of bandwidth. The difference is that each port can handle the full bandwidth, so adding additional USB devices will not share the already limited bandwidth.
With devices that are designed for USB3, like the RSP, this may not seem like a big deal. However, that depends on what other USB devices you are attaching to your PC and how much bandwidth each requires. Many USB devices, like mice, keyboards, etc., use very, very little bandwidth. The big bandwidth hogs are devices like SDRs (no big surprise there) and hard drives.
So, my suggestion is to look at your system and what USB devices you have, and how much bandwidth each device requires, then decide if you need to spend the extra money on a card with multiple processors for multiple ports. BTW, I picked those cards somewhat at random, so don't look at those specific cards as recommendations. However, just FYI, I have 2 of the 4 port StarTech cards in my system. Since implementing those cards and disabling the motherboard USB3 ports, I've had zero bandwidth related issues with SDRs on my system. I do have a few SDRs that are USB3 and use every bit of the USB3 bandwidth on a port. My SDRs I'm referring to have bandwidth of 56MHz. I still use my motherboard USB2 ports, but for things like mice, keyboards, etc. Using a high end USB3 addon card for a mouse is quite a bit of wasted resource.
Also, the Renesas USB3 chipsets are known to do a good job with their downward compatibility to USB2. Before deciding what USB card to get, make sure you do some research to see what other people experienced with the particular card you are looking at with respect to its downward compatibility.
Another thing to note is that the bandwidth of the USB card / cipsets is not the only factor that should be considered when dealing with optimizing USB bandwidth. Different types of PCI Express slots have different bandwidth. While no one PCI Express slot should be a limiting factor, it depends on what other devices are sharing the PCI lanes. I know this is getting a little detailed, but not all PCI Express slots are created equal. In the documentation for your motherboard, you will see "PCI Express x1", and/or "PCI Express X8", and and/or "PCI Express x16". You may have to read the appropriate sections of your motherboard manual to understand how these differ and whether or not they may even have any impact in your particular configuration. Here's a decent reference on PCI Express:
https://en.wikipedia.org/wiki/PCI_Express.
Finally, and maybe the FIRST thing to check is make sure your drivers are up to date. Old drivers can have a performance impact vs newest drivers.
Your combination of a single core PC, running 32-bit Vista and only 4GB of RAM is going to be an issue with anything other than minimum bandwidth settings on the RSP. The general rule of thumb I've always heard is to get as much computer as you can afford. You don't want to buy a new one only to find you've maxed it out with increased bandwidth on the RSP. You want spare overhead if for no other reason than to not have to buy another PC a year down the road.
A few final comments. You mentioned an external hard drive in your post earlier. Do NOT run a SDR and a USB external hard drive on the same USB controller. That's asking for trouble because they're both high bandwidth devices. If you do get a PCI Express USB card, I'd suggest one with multiple processors, and a USB3 vs a USB2 - one processor per port if you can afford it (like the example I noted above). That will give you the most bang for the buck with respect to bandwidth processing. Next, if you have to make a tradeoff between a faster processor and more memory, more memory will give you more return. Next, when you're setting up your new PC, definitely DO READ THE MANUAL sections about the different PCI Express slot types, again, as I noted above. If you have an addon graphics card, it should go in the x16 PCI Express slot. If you do not, and are using graphics on the motherboard, and have an x16 PCI Express slot, use it for the addon USB card. That slot, vs the x8 or x1 will give you the best bandwidth and mitigate any potential USB data transfer issues (from the USB card to the motherboard).
Sorry (again) for the length of this post. I was trying to give you some additional detail to help you troubleshoot your issues and maybe help you with some decisions for purchase of your next PC.
Best regards - David