Going into this fall I had it in my mind that I was going to build an entire new machine by now. As time went on and I did research I just wasn't excited about the possibilities. I currently have a 2.4 GHz Pentium 4 C Northwood (512 KB) CPU overclocked (stabily) to 3.0 GHz. The best available now is a Prescott running at 3.X GHz and 1 MB of cache, and quite expensive at high end still. At this point I am waiting for Prescott's with 64-bit and 2 MB cache as that will be the point where features and performance will be worth the upgrade.
Instead I have done some upgrades to my current machine that are pretty good bang for the buck items. I got a XFX geFroce 6600 GT AGP 128 MB card for a pretty good price. This was a major upgrade video wise from my old geFroce 4 4400 TI. I have been enjoying it in Half-Life 2 and haven't had chance to test it with Doom 3. The second upgrade was an additional 512 MB of PC 3200 DDR ram from Crucial. The last upgrade was a Pioneer A08-XLB 16X DVD burner with 4x DL burning.
The only other things I am considering right now are some new hard drives. Something like a 36 GB 15K SCSI drive from Seagate and a 74 GB 10K SATA drive from Western Digital. Also maybe a nice CD-Burner before they all disappear.
Try moving to a terabyte or two first.