The learning curve for moving to USB is pretty steep, but you only have to do it once. It helps if you can provide us what kind of data bandwith you are after.

USB low speed is OK for human interface (switches, LEDs) or other slow control (temperature). I think the fastest data is about 8 bytes every 10ms so you can compare that with what you are doing now.

Full speed is better for more/faster data. Unlikely you would need faster (high speed) if you are replacing RS232.

If you want to stick with 8051 then use the Keil CPU selector chart. Most vendors, 8051 or not, will supply some basic templates to get you going. Cypress and SI Labs come to mind. Atmel perhaps.

Check out Jan Axelson's website for more information (lvr.com?). If you stick with an HID class device, regardless of speed, you can write a program on the PC side that is application level, so you don't need to write a low-level driver for it. My only experience on the PC side is C/C++ so I don't know what other languages could be used.

Certainly the FTDI solution is the easiest and will be price competitive unless you are moving many, many units. And it will work with many OS's. If you roll your own and need Linux or Mac support too, then it gets to be a very large project.