First and foremost Welcome to Hexus!
You've done the right thing by starting to look at prices and educating yourself about hardware. Then you've done an even better thing and posted on Hexus . There are many knowledgeable people here who will get you good value for money.
On the 20th January Intel will launch their new core 2 duo cpus (the quad cores will launch in Feb/Mar) so it's not a bad time to think about buying. I think it's safe to assume that your main use for the PC will be gaming. Do you do anything else like media encoding, music, film or any other specific use?
You've already mentioned that a 19" or 21" screen is what you have been looking at. Do you intend to play movies or watch TV on this screen? There are lots of good monitors around but the choice and size of monitor will be determined by what you want to use it for. Will you need dual screens at all?
With regard to gaming the choice of graphics card is quite closely linked to the monitor and it's available resolutions. There is little point in spending lots of money on the latest graphics card(s), then using a monitor at a resolution that isn't going to benefit. An 8800GT/8800GTS 512MB graphics card will give similar results upto and including a resolution of 1680x1050 (20"/22" widescreen monitor) when compared with the more expensive 8800GTX. As you up the monitor resolution (which usually happens with screen size) past this then the 8800GTX starts to pull ahead in gaming.
I'm not aware of any machine that will run Crysis at full settings on 22" screen at a decent frame rate. It's a bit of an anomaly for a game.
Do you use headphones or a surround sound system for gaming and music?
With regard to RAM then DDR3 is not needed. It's extremely expensive and the current crop of processors will not see any noticeable benefit from it in the real world. Better to spend the cash on 4GB of quality DDR2. If you are not overclocking then PC6400 will be fine.
CPU choice is a bit more complicated and depends on what you use the machine for. People who often use programs that can utilise more than 2 cores tend to go with the quad C2D Q6600. It's very good value for money and overclocks well.
I'll leave it at that for the moment to let you digest the thread and the suggestions.
If you can give us as much information as possible with regard to what you are going to use the PC for and what type of performance you want then we'll have a better understanding and therefore give tailored advice.
I've always found that it's much better to gather all the information possible and make an informed choice when spending the moolah, so take your time and resist the urge to get your mitts on shiny new kit ASAP.