If u are familiar with the computer parts. Yes, doing it is fun
and no problems. Sim Lim Square (SLS) is the best place to shop for
parts. I can spend the whole day there just collecting the
pricelist and do comparison and end up buying wat I want the
following week 
If u are not, please do read up first, there are many good books in
our local national/regional libaries or the internet that provide
such information. I have come across many incidents, ppl brought
the wrong parts, went back to the store to make a big hoo-ha out of
it which in the first place they are in the wrong (failing to read
up).
(example: buying a top of the range PCIe video card for their 3D
gaming home to find out that their board only support AGP which
they should have found first before purchase.)