Hi eric.... I also have a QS 733 and have been slowly upgrading it over about 5 years or so. The Memory upgrade was very simple. As stated make sure its PC133. I just unplugged my system, plugged in the new memory and powered it up. Go to 'About This Mac' and it will tell you what your new memory status is.
I added a wireless card PCI version. Again simple plug and play. I didn't pay a lot for the card either which made it even more satisfying.
I added a new Superdrive (£25) on ebay. New in box etc. Again only armed with a screw driver it took about 10 minutes to instal and power up. QS recognised the drive straight away.
Have just bought a new Hard drive. Paid for a Seagate Barracuda 120Gb and was sent 160Gb (although it will only format to 128Gb on my QS). Set as SLAVE so I can keep all my data and just move it across to the new drive. Cost, with screw set was approx £45).
All the above over a period of 5 years or so. I am looking at Processor upgrades and the 1.8Ghz Sonnet looks promising but a little expensive so will be waiting a while on that.
I still have the original Graphics card NVIDIA GeForce2 MX Twin view AGP with 64Mb. Can upgrade this quite cheaply, although it runs my system quite nicely.
QS really are a great way to upgrade from as it is all so relatively simple.
and I know people will say for the cost I could have bought a new better, faster, all singing system..... but that would have meant one large layout of cash.... upgrading has allowed me to do it over a period of time and my bank account hasn't even noticed money is missing.
Good luck with your upgrades and enjoy the process.
Reaperman