Your best bang for the buck would be to upgrade your video card to an ATI 9800 Pro for about $200. Beyond that, maybe pick up another 512MB of RAM. Those two things would keep you going for quite awhile.
Are you talking an entire new computer, or just upgrades to make to the one you've got? If all new, then you should really go Athlon64 for gaming. If just upgrading, I'd start with at least 1GB of Corsair RAM. Next I'd look into new video or sound card (depending on which you think you need most).
Funny, because this is pretty much what I said over on LCVG.
Dome,
I don't think you need a fullblown upgrade yet. I think a fullblown upgrade is premature at this point. I think a 2.4 P4 is fast enough at this point for PC gaming.
The real performance limitation these days tends to be video cards. I would just upgrade your video card to a 9800Pro - or if you really want to catch up with the technology, get an Radeon X800 of some sort. Personally, whatever the Doom3 advantage of the 6800GT is, it's not worth the giant card. Plus, the X800 is faster at some other games.
Seriously, just get a new video card and another 512MB of PC2700. Don't worry about the CPU yet.
I'm still running an Athlon XP 1800+ and PC2700. Although I've considered a CPU upgrade (I have a refurb Athlon 64 board I got for $30 on a deal), I'm just looking to find a good deal on a video card at this point. My Radeon 8500 is about the same era as your Ti4200 - it is definitely the weak point.
I think I made a mistake by not saying I have no desire to upgrade the CPU. The changes I was looking at were power supply, video card, and memory.
Thanks for the replies folks. After doing some research, the new ATI X800 looks like the best longterm deal. While it may not play as fine as the GeForce 6800 for Doom 3, it still plays it in high quality, and it'll be good for Half-Life 2 as well.