Good advice Jerome. You have a couple of drawbacks with your current system.
1. You can only record the total mix from your mixer. ie: one track at a time or all tracks as a single main out.
2. The incoming audio to your mixer is converted four time before you can hear it. Audio to digital ( within the mixer) - digital back to audio( for the rca main out connection) - back to digital to get it recorded by your computer and finally back to audio so you can hear it. Not all convdersion work at 100% so you are best to keep the conversions as low as possible.
3. You are using the sound card within your computer to do a lot of work and they are generally not up to the same spec as the sound card in a dedicated mixer.
4. You will mosty likely experience latency issues when recording and listening at the same time. Latency will appear as a slight dealy in sound from what you are recording to what was previously recorded. This is caused by the speed of the various audio converters coupled with the number of conversions taking place. Latency should be kept as low as possible.
Now, the advantages of a USB connected mixer is tremendous.
1. Only two conversions take place. Audio to digital ( performed by your soundcard in the mixer ) - This signal is then passed to the computer and recorded without any other conversion process. You may use additional software plug ins but in effect they are are still only acting on digital to digital processors so no actual converting. The second conversion takes place after your computer has passed the signal back to the mixer via the usb connection where it is converted back to audio by the same soundcard that converted it in the first instance. Two conversions is the minimum number of processors you can have when recording digitally.
2. The quality of the soundcard in your mixer is most likely better that your computer. Both conversions should be better. More headroom - less noise - faster
3. Latency should be so low as to be irrelevant.
4. You can now record multiple tracks simultaneously by setting the input on your daw to the mixer track you wish to record.Each track will now appear as its own track in your daw and can be acted on independently.
IMPORTANT NOTE : To keep latency as low as possible the transfer rate from the mixer to the computer must be kept as fast as possible. In order of speed from slowest to fastest they are:
slow: USB1 - latency issues here , avoid
firewire - Neglible latency butnot as convenient as it is older tecnology and there are are numerous firewire connecters available. Need to check on the cable and computer firewire connectivity before purchase.
Fast USB2 - marginally faster than firewire but with the total easy of USB.
Fastest - USB3 - dont know if its out there yet but if not it should be available soon and will be a big step up form USB2.
Hope this helps
Good luck with what ever you do.