My PC doesn't have a HDMI input, so whats a good capture device for USB

The "gold standard" for recording AV output (which is native to the N64) is generally considered to be the GV-USB2: https://www.amazon.com/DATA-connection-video-capture-GV-USB2/dp/B00428BF1Y

It's used by a bunch of runners across all N64 communities and gives good quality. I'd recommend also looking on sites such as eBay to look for better prices, though do be careful and make sure you get the right one; a lot of generic and worse capture cards will have "USB 2.0" in the name, which doesn't have anything to do with the GV-USB2 and simply refers to the connector it uses (I fell into the trap of getting a crappy "USB 2.0" capture card myself, which stopped working in two weeks). After you get a capture card, you'll need a way to split the signal coming from the console so that it can go to both the TV and the capture card (which is connected to your PC). Although you can technically play from just the capture card (which you'd record in a program like OBS), this usually comes with massive delay (at least a tenth of a second) and is not recommended. The best way to split the signal is by getting a splitter cable.

What you'll ultimately need to acquire:

• Capture card (GV-USB2 recommended)

• 3x female to 6x male splitter cable such as https://www.amazon.com/BronaGrand-Female-Splitter-Adapter-12inch/dp/B07T8HKZ5H

• An extension cable (length depending on how far your PC is from your N64/TV) such as https://www.amazon.com/TNP-Extension-Composite-Extender-Connector/dp/B072N1XZRZ

There are quite a few steps so it can be easy to feel intimidated, but everyone here is willing to help out. After everything is set up, you'll need to install the drivers for the GV-USB2 from https://iotku.pw/gvusb2-guide/ and then add it as a Video Capture Device in OBS. We also recommend changing the capture card's Deinterlacing settings in OBS to "Retro" for better video quality.