Some of this isn't going to happen. The S478 P4 is good up to around 3.73-3.8GHz. 4GHz isn't likely. The prescot S775 P4s could do 4GHz+, but I doubt you have one.
Source games like L4D, TF2, etc should be fine. Any game that has a dual core as the minimum won't really be playable on this rig. You will also have the issue of spending a price premium for the board, ram, etc. The cost of doing this really isn't worth the benefit.
I found a very similar computer like 2 years ago. HP D530. I got an old 3850 agp card on ebay for $50. Slamed it in along with 4x512 of ddr 400 and 2 WD 80gig IDE drives in Raid 0.
Unfortantily those cards are hard to find now.
That computer gets a 3d mark 06 score of around 6000. As a point of refrence at the time i had a q6600 core2 quad at 2.4. with an 8800gt that computer got 11000.
My p4 can play MW2, World at War, MW3 at a solid 45 FPS with everything on low and of course aa off at 1280x1024. It cant run Black OPS or BFBC2 with consistant frames.
It can max out all source games.
My HP Mother board had no over clocking ability and very little bios adjustments. So I cant speak to how an OC to 3.6 or 3.8 might help.
When my bro comes over he uses it and we party for MW3 he is always in the top 3 on it, he says it does not lag or anything.
If your goal is to learn something then go for it. If the goal is to have a useful computer in the end you might be waisting your money. A better project would be to see how much of a gaming system you can build with 400 and the parts from that old Dell.
Maybe get an i3 for 100 a cheap board for 50 ram for 50 a gpu for 100 case and psu for 50 and reuse the Dell hd and optical. In the end you would have a computer that has a future. I found the more I messed with the older computers the more disappointed I was in the lack of performance I got.
I wouldn't drop that much money on a platform of that age, you could look on craigslist for older parts that people could be getting rid of if you are interested in playing around with an older platform.
And unless you are really rich I can't see spending $400 on something that will always be slow, and tied to a system that is 5 generations old. The ram, motherboard, GPU, etc will never carry over to a new system.