I'm sure this has been covered before but I can't seem to find anything in the search.
I've been using an M-Audio condenser mic with an Edirol FA101 preamp for years, but recently I've been getting buzzing and hissing artifacts in the recording. I figure it could be the FA101 unit or the mic (I get the same problem with an AKG C1000s, but not with an entry level Shure), so I thought it might be a good time to take a dive into the USB mic market instead of changing the FA101.
I quite like the idea of NOT having to plug in a firewire device everytime I get the urge to record a quick idea, so I thought a USB mic might do the trick. I must admit, I always thought USB mics would be substandard, but I see all the big names like Shure and Rodes are doing them now.
I'd love some advice. Would a low-budget USB mic be about the same quality as an M-Audio Nova that is now 15 years old.
Thanks for any help.

I haven't looked at all USB connected mic's. But what I have seen is designed for Podcasting and speech, not singing.  Based on that I wouldn't think a low end/inexpensive one would suit you.
 

burgerproduction
..I've been using an M-Audio condenser mic....I've been getting buzzing and hissing artifacts... I get the same problem with an AKG C1000s, but not with an entry level Shure)...

"Entry-level Shure" suggests a dynamic mic, so if that's working fine but the two condensers are noisy, then it sounds like your interface's phantom power circuit could be going out.
 
If you want to test this theory, you can get a phantom power supply (single channel) for as low as $25:
 
https://www.sweetwater.com/store/detail/PS400--behringer-micropower-ps400
 
Alternatively (and even better IMO) you could use a standalone Mic pre, connected to your interface's line input.  If you don't already have one, something cheap like this would do just fine:
 
https://www.sweetwater.com/store/detail/TubeMP--art-tube-mp
 

Bear in mind also, that with a USB mic, the mic is also your interface and your mic pre.
 
So you've got absolutely no flexibility as far as using another interface (if you're using ASIO, you're going to have to disable your Edirol as an ASIO input device), and you've no choice in using different mic pres.
 
Also many USB interfaces (especially laptops) cannot deliver the power required to power these mics. I tried out the Behringer C1U, and apart from being a pretty naff mic, it required a powered USB hub to even get a decent signal.
 
JonD's suggestion of the ART Tube is a good one.
 
PreSonus also a similar offering.
 
Alternatively, take a look at replacing your Edirol with something like a Focusrite Scarlett. The mic pres on the Scarletts are great.
 
I've got a Yamaha 01X, i88X, DS2416, a Behringer ADA8000, and 3 Focusrite scarletts and the Focusrite's blow the other ones away in the Mic pre department.  I've also got 2 x GA Pre-73 neve clones (one upgraded with Carnhill transformers), and the difference between the Scarlett and the Pre-73 is negligible (unless I'm purposely pushing the Pre-73 to get that Neve type distortion).
 

Thanks everyone,I'll keep a look out for Focusrite soundcards offers (I'd been thinking along that route anyhow),
and I'll test out the mic with a tube pre-amp (BEHRINGER MIC100) I bought ages ago.
I've owned an Art Tube pre-Amp before, but I kept getting recording artifacts and background noise.
The whole unit packed up pretty quickly; seemed a bit delicate.

Thanks everyone for your help.

USB mics are made more for "PodCast" recording than more traditional instrument/vocal recording.
The USB mic (as has been mentioned) is also the audio interface.
That's the weakest part.  The quality of the audio interface and its driver.
 
If you're wanting to minimize noise, that starts will avoiding ground issues, using balanced cables, proper gain-staging, etc.  You can carefully go thru your entire setup... looking for sources of additional noise. 
ie: A poor quality USB cable can cause noise issues.
Even with higher end gear, you still need to be mindful of ground issues... and sources of noise.
Electrical appliances, dimmers, fluorescent lights, electric blankets, fans... can all cause noise.
You can minimize the potential for ground issues by powering your entire setup from a single outlet. 

I've just had a thought. Could I be getting noise issues due to bad grounding?
My laptop has a 2-pin power cable and so does the Edirol.
Is there any way to ground them?

You could absolutely be experiencing a ground issue.
Try running the laptop off battery.
If the noise goes away, you've got a ground loop issue.
There are numerous solutions/workarounds
 

On the back of the Edirol FA101 is a grounding screw (same on the FA-66): https://www.google.it/url?sa=i&source=images&cd=&cad=rja&uact=8&ved=2ahUKEwiYpK2ft8_eAhUDTcAKHcm1BTkQjRx6BAgBEAU&url=https%3A%2F%2Freverb.com%2Fitem%2F498088-edirol-fa-66-digital-recording-interface-firewire-audio-capture&psig=AOvVaw1OwVyt20CR6CzIcS1prPE8&ust=1542131918229437
Could there be some way to ground that to 'something'?

Little update.
It is the M-Audio Mic. I did some tests with my tube pre-amp and the M-Audio Nova, then my AKG C1000S and found the Nova to be giving off noise. Should I even bother trying to fix it?
I've got some backup mics (AKG C1000s, AKG PS5) but I'd like a large diaphram mic to replace the Nova.
I've seen that there is a Behringer C-1 for as low as €26 or Audio Technica AT2020 for as low as €79
Any suggestions?