I put in a lot of effort painting a bike as you describe, i.e. properly given non-specialist materials and lots of elbow grease. However, for the effort, I do think it is worth removing the bottom bracket and headset cups. (Or getting someone to remove them for you.)
I was traumatized when my freshly painted-in-yellow bike fell over for the top tube to scratch its way down a stone pillar. That made me think 'never again' to spending a whole weekend and a week's worth of evenings 'getting it right'.
For my next attempt I went for stove enamelling. This was with a local place and the price was a bargain. They did not specialize in bikes but they had no problems with my frame/fork. The look was good and I had not had to wait long, just strip my bike down, and on return 3 days later I could build it up again.
Stove enamel is different to powder coating. You can look up the pros and cons of stove-enamel vs powder, personally I think that it depends on the look that you want. Other than that enamel is easier to maintain as it is T-cut compatible with no clear-coat needed.
If you want a bike that looks different then there is a lot that you can do with the stove-enamel paint job. It has a different shininess to it, verging on matte.
With some things it is better to do it yourself if you want a proper job. This is not the case with paint as people that do it everyday have skill and experience that your extra effort and attention to detail is not going to make up for. It is like plastering - all considered it is better to get a professional in.
There are other strategies for painting a bike. You can give it a good clean and slap loads of Hammerite on it with a brush. This will result in the distinctive Hammerite finish - usually reserved for garden gates and tin sheds. Totally bulletproof.
On another bicycle I liked the paint job but I knew I would be kidding myself if I tried to touch it up. So I used a complementary colour. With a wire brush I cleaned the new chips and quickly put the touch up paint on every time I refurbished the bike. This resulted in an 'art-is-tic innit' theme that people complimented me on.
If you want to go for something really unusual you can go for nickel plating. This is like chrome but better when it comes to rust, particularly if you clearcoat it too.
On balance powder coating is better than stove enamel: http://www.windridge.co.uk/content.php/412
Also of note is that a professional paint shop can shot-blast the paint off your bike in minutes, to a better result than spending all weekend with wet-or-dry paper.
My recommendation: posh bike - get the pros to do it. Heap-of-junk, touch up with complementary colour(s).