get a video card with sm3.0 capabilities. if you have any hope of upgrading to vista, buy a new video card in a few months from either ati or nvidia, but dont buy any current offerings - IF you want to goto vista.
suggest pci-express grfx - if you have the capability to, if not suggest a new logicboard upgrade with pci-express.
keep in mind these new video cards to be released - IF you get the higher end ones - they require a special grfx connector for them - so this may require a new powersupply - nothing you have to worry about im sure they made an adapter for your old power supply tho.
other than that, you can get amd or intels core2duo - both are good, but i do belive intel has a leg up until amd releases in mid year... you can wait to see what amd has or buy intel now.
SATA ii standard is a great hdd to get. i went with a raptor hard drive from western digital, its hot and its loud but performs those games WAYYYY BETTER than anything else. anyways, i suggest for people who dont want the noise and heat to buy a parrallel platter hdd - i forget if this is the name im looking for here - but it puts the data on the platters so it can put more data on each. in the same ammount of room.
get some good memory, none of that value mem crap, and get at least 2gb's of memory - it can be had for around 200$ which is pritty cheap (for ddr anyways). if you get DDR2 then it will likly be more expensive, but get something with good timings - depends on the motherboard you get tho.
i will be looking for some parts up for a client - if you want lateron when i come up with them, i will post what im getting. his application will be differnt than yours but it will do your stuff too.
as for your PSU - with newer technology you are looking to be using more wattage per device - meaning your DVD+-RW 500gb hdd and video card will likly take up alot more than you thought before, not to mention the new logicboards that support newer cpu;s which hog alot of wattage too. alot of people seem to do great with about 350watts without any issues with a nice setup with a high range vid card - next gen vid cards are going to be really hogging of wattage, i belive the new ati video card will be around 200watts (the high end card) or so (give or take, but i know its very high - i dont have the exact figure).
https://secure2.pcpowercooling.com/cgi-bin/pcpcorderform.cgi <-- some affordable too!
PSU's are made differntly! your old dell 200w psu might be great but i been through alot of psu's and they are not all made equaly. i hate to admit it, but i run ALOT of power through my system and since i have my own place now i see the bill i have to pay when i run my computer - its insane, i cant belive i went through so many power supplys but now i can see why - i stick with PC power and cooling because i havent had it fail yet! see the reviews and tell me what ya think - alittle pricy and mabe its not your thing - mabe you dont need the power it provides, and mabe if your system isnt as taxing you wont have to worry about the quality as much. but the new hardware out there is more demanding especially when adding SLI and various of new technologys from cpu manfuactures and vid cards.
good luck with it.