I'd go for it if I were you.
As others have said, start off part time, keeping your existing job, and keep overheads to a minimum. You may well find you can undercut the competitors due to working from a spare room and low overheads / lack of offices/staff to pay for.
Work out what you need to be able to make a year, then add tax, and all the costs involved, plus a bit extra in case of emergencies, and then divide that by 52 – if you think you can reasonably bring in that much a week, then you've got a good chance.
Also, when you start out, doubtless friends/family will use your services/buy your products – discount this from any projections of earnings, as 90% of the time this is them being nice and supporting you, and won't be an ongoing thing.
Look at what your competitors are doing – find ways you can improve on what they are offering. Also work out who your target market is, and how you are going to reach them – contact any potential customers now, and find out the situation with their current suppliers – they might be convinced to use you instead if the service/products they are receiving is not up to scratch.
Oh and get *everything* in writing – no matter how small the order – comes in handy in cases of disputes.
I started doing websites about 12 years ago, did part time for about 5 years, 3 of which I was stacking shelves in a supermarket.. I then left when I had enough work, and things have been going pretty well ever since.
I've never met anyone who regrets having a go at setting up on their own – even the ones who failed are glad they gave it a shot – and as long you don't invest all of your savings in it, and risk everything – then you should be ok.
Hope this helps – all just my opinions