SX2870... that sounds strikingly familiar. I think I may have purchased one of those, with a Sandy Bridge Pentium G630 CPU, and then re-sold it to a friend's relative.
Was it a slimline SFF tower?
I would advise considering to replace the whole thing. Get an Asus (Acer?) i5-7400 / 7500 Kaby Lake quad-core, with 8GB / 16GB of RAM, and preferably, an SSD. (They sell them now in OEM boxes.) Or get the HDD version cheaper, and install your own SSD. Re-installing Windows 10 with a USB stick is relatively a piece of cake. You can get i5 CPU towers with 8GB and a HDD for as low as $399.99 at Newegg, brand new.
I would advise that over purchasing just a i7 CPU upgrade for your existing unit, unless you happen to be rather technical. Upgrading OEM boxes with higher-end CPUs,
if they were never sold with those CPUs as an original factory option, is fraught with peril. (VRMs not specced high enough, BIOS issues, etc.)
Ryzen's not bad, but it's rather overkill for web browsing.
TBH, I'm surprised that your web browsing is slow. Could it have to do with the number of add-ons?
Do you currently have an SSD? That alone might breath new life into your PC.
Btw, TopWeasel, an i3 has 4 threads, it's very unlikely to get thread-locked.
Edit: Btw, if you upgraded your video card "a few years ago", then it probably doesn't support hardware decoding of the newest video formats.
Scratch my earlier suggestion, consider instead, NOT upgrading the CPU, but instead, installing a (240GB or larger) SSD, AND an NVidia GT1030 card (they JUST came out). If you need a single-slot, or low-profile, or both, they have those too. (Probably essential if you have the slimline model.)
I put an NVidia GT430 (128-bit RAM) LP into my slimline Gateway, before I sold it, and upped the RAM from 4GB to 8GB. The new video card helped immensely.
And to think, the GT430 is considered "old" now. (Fermi generation). The GT1030 (Pascal) is the new hotness for SFF / video-watching.
If that's what it is, and I suspect it
could be the case, is that the videos you are trying to watch online, are NOT decoded by the video card, but instead, by the CPU, and that's bogging everything down.
https://www.newegg.com/Product/ProductList.aspx?Submit=ENE&IsNodeId=1&N=100007709 601297382
https://www.newegg.com/Product/Product.aspx?Item=N82E16814487347
Note that these newer Pascal cards from Nvidia, as well as the Polaris cards from AMD, do NOT support VGA output any more.
You would need to buy a DisplayPort to VGA cable, Newegg had a Rosewill branded white one on sale for $10.99 recently, I almost bought a few, just in case I needed them.
Edit: Looks like this is your system?
https://www.newegg.com/Product/Product.aspx?Item=N82E16883113220
Mine was similar, only with the Pentium and not the i3. Probably same motherboard. Sadly, that board only has two SATA ports on it, and not four. Which means, that you would have to actually replace the HDD with an SSD, and not just add it and keep the HDD for data. Which could make things slightly more complicated to move your data around.