ok so here is what i decided to do when i realized that it wasnt worth it to upgrade my dell.
-motherboard: gigabyte p35c-ds3r (compatible with DDR2 and DDR3 ram) $180
-Ram: 2x1BG dual channel DDR2 (i hear DDR3 is not yet running to its potention and DDR2 ram is inexpensive) $130, but go as low as $80.
-Processor: core 2 quad q6600 (paid $50 for the upgrade from a dual core to a quad core) $280
after that i paid $50 for a case and $100 for a suitable PSU.
-video card: havent bought it yet, but nvidia 8800gts 640mb will last me along time for $350-400. i was in the market for a gaming PC and didnt want to have to upgrade it for several years. i kept my drives and hardrive. this setup will run the newest programs at the highest levels for a long long time.
if your short on money however, this is what i would recommend.
-motherboard: Intel® Desktop Board DG965WH (dd2 and core 2 duo compatible. cant run a quad core, but doubtful that u will be needing one anyway since the duo's are excellent choices and fast enough for any game.)
http://www.newegg.com/Product/Product.aspx?Item=N82E16813121048
this board is about $100, the linked one only runs ddr2-800. that isnt a problem, but if u want one that can run any ddr2 u can find slightly modified versions of the same board for $10-15 more.
- processor : the one in your current dell system (intel says the board will run any P-4), so u dont have to buy a new processor untill your ready! and there are 3 newer models of processors to choose from that will work with this board.
-memory: u only need 1gb memory as it is, however i would recommend getting 2 1gig sticks of dual channel ddr2 (2 gigs is enough to run even the most damanding video cards, and other hardware). this will run u anywhere from $60-150, u can spend as much as a grand if u really wanted to tho

-video card:
http://www.nvidia.com/object/graphics_cards_buy_now.html take a look here at nvidia site, the 7100 gs is only $55, has directx9.0c (it can run any game on the market currently).
-from here u can recycle yoru hard drive,disk drives and probly even your old case and PSU(these specs are fine with a generic PSU, but if u were to upgrade your processor or put a top end video card in u would want to buy a reliable PSU, i put an enermax in mine).
http://www.extreme.outervision.com/index.jsp
the link is for a power supply calculator to figure out how much u need
so total here u would be more than $250 and your computer should be able to run vista. it might be slow as it is, but you have a lot of options for upgrades and could turn it into a very compitent gaming PC if u want to later on and certainly beats trying to upgrade your current build.
i hope this helps you make a decision
swen