Lets start you off with a nice Intel Core 2 Duo E6300
. That should hold you for a year or 2, maybe even 3.
Next, for the video card, I'd get a Nvidia 7600GT
. Yes, it is outdated with the DirectX 10 era dawning, but it should be able to play Battlefield 2142 nicely.
For memory, you don't need top of the line DDR2 800 unless you're overclocking, 2 GB of DDR2 667 Corsair ValueRAM
should do quite nicely.
For the motherboard, slap all your parts in an Intel DP965LT
. Again, not good for overclocking, but it should be able to do justice if you want stock power.
Stock power is very good as it is, but some will encourage you to overclock since it seems like a 'waste' of a processor's potential. Yes, it could increase a little performance, but mostly in benchmarking, and you probably wouldn't notice it in real life situations.
Also, it voids the warranty on the processor (And some RAM), so if your processor goes kaput (not likely if you overclock carefully), you're on your own with no help from Intel.
You can reuse the hard drive, CD-ROM drive, monitor, and maybe the case if it's an ATX form factor from your old PC, but you may need to install a fresh copy of Windows on there when you're done upgrading.
You may also want to sell that stuff to make up for a little of your upgrade.