In the long run, it is more expensive and less beneficial to upgrade peice meal fashion. And then also you have to contend with future developments - what might you miss out on? And what things will be made incompatable?

Lets say for example you decide to spend that $100 dollars on a new motherboard in anticipation of being able to eventually upgrade all other item to that board. What will hapen is, assehole hardware developers will ensure the new items developed in the future, wont go on that perfectly good motherboard! Or you may see another motherboard released in the future that could make you regret bying this one!! This scinario can happen for any other item of hardware as well... eventually when you do have to change your motherboard, who knows which item you buy today, will not be compatable?

Last time i bought a computer, i got the best deal off Ebay, complete system's minus the monitor seem to provide the best value for money these days.

You don't have to be on the cutting edge to get decent performance though.  The problem of new hardware arriving which is incompatible is always going to exist, whether you buy a computer a piece at a time or all at once.  What you need to do is decide what timescale will be involved in getting all the compatible hardware that you will need, and save your money until you can buy two or three things on your list.  It also doesn't hurt to research motherboards (you can usually download the manuals for these) and determine if certain graphics cards are not compatible.  After all, the last thing you want is to boot up your newly-upgraded system and get a blank screen.

Once you commit to buying a particular motherboard then that does largely decide what other parts you'll need or want.  And if you're particularly budget-conscious, you can wait a bit for the prices of those to come down.  Just don't wait until stocks completely run out, as hard-to-find components can become pricey.  Case in point, DDR2 sticks are usually cheaper than old sticks of DDR of same capacity.

Good advice, and i will add this much as well -  if you think of it in terms of the 'best performance for money' ratio, cutting edge technology is the last thing you need! The best performance for money ratio can be found with aproximately mid to mid-upper range technology.

You may be able to by what they call a 'bare bones system' which you can then fully complete using your left over outdated equipment. But most likely your old RAM will not be compatable, that is usually the first thing to drop off the compatability cliff i'm afaid.

Actually if you dont have a sound card, get one of those, I was VERY surprised how much a cheap soundcard helped my audio.

Really a sound card is only a good buy only if you have decent speakers and/or headphones.

Personally, If I didn't have a very specific goal in mind for my upgrade, I'd save my $100 and put it towards a complete overhaul at a later date. As far as whay Moosetek13 says, I have to disagree to a certain extent. While $100 will not give you an overall leap in performance across the board, putting you're money in a specific place to accomplish a specific goal can greatly increase performance in that one particular area.

For $100 the only aspect of your machine I'd consider upgrading would be the video card (if you aren't satisfied with you current performance in games) or the sound card (if you have a fairly good set of speakers or headphones). Otherwise save your cash for the future.