I'm looking at either buying a new 550d or modding an antec300.

I was wodnering whether modding it will yield a good enough silence performance? if not then I'll just get a new case.

here's a case mod on youtube of the antec 300.

http://www.youtube.com/watch?v=4aYsZzI1aMQ

If i choose the mod route I will need to buy a dremel and various noise dampening materials, it could cost me around £60. ($90) all in all. But I guess the dremel would be an investment and not a sunk cost.

Do you want to spend a bunch of time and money in a mediocre case to silence it?

If so, buy the 300 and mod it.

If not, buy a 550D or R3 or P280 and spend the money and time you saved from modding to buy good fans and a fan controller.

Hi, as I understand it you already have the Antec 300 case so...

Two questions:
1) the proposed spec of the system you're going to build.
2) do you want the challenge / reward of making something unique.

It looks to me like the airflow of the case is already good and yes it probably doesn't have much/any noise blocking qualities but...
If you're building a modest system then you can put minimal number of fans in the case and run them very slowly. A really quiet PSU with a 500rpm case fan and 500rpm CPU fan will be very very quiet, and you could still have quite a lot of power this way. Core i3 or i5 under a big tower cooler can be cooled with 500rpm fan and modest passive video card. If you don't make noise you don't need to block/damp it!

For the hard drive I would put it on it's side in the hard drive bays, either sitting on foam (I did this with good success) or suspended. If you have several HDDs then more of a problem but think laterally, or outside-the-box. In my case there was room for the hard drive in Scythe Quiet Drive and big video card so I cut hole in the case floor and routed the cables out and it sits on foam next to the case!

If you're building a high power and/or overclocked machine then a good free-breathing case with sound damping would be helpful, but it's still about airflow management and most importantly fan speed/control.
Seb