First of all a couple things:
1. Are you just gonna throw the new CPU/RAM/Mobo into your current case? If so, make sure your case is an ATX 300W. If not then add $50-100 for a new case
2. DDR is the new type of RAM and my question is, do you just have 2 sticks of 256MB of PC133? The reason this matters is because there is one mobo that has 2 DDR slots and 2 SDR slots.
Ok now to my thoughts. DDR increases performance significantley in current generation DDR chipsets (by 20% especially in games), and I guess, starting with SDR is ok, but it limits your mobo options to one (ECS K7S5A, the one mentioned above) and I would really just sell your PC133 and get DDR. K7S5A, well, it is a sticky subject. Some people love it, think it's great, but others have had nothing but trouble, so I guess I would for a newbie like yourself, have to hesitate to recommend it. Other people will say that VIA KT266A boards is no better than K7S5A. I have not had any of the issues people have had on my VIA based system (not KT266A, older), so I guess it's up to you if u on which to go for. I personally think that you have a better chance of having a trouble free upgrade by going a Asus A7V266-E, or EPoX 8KHA+ or MSI K7T266 Pro2 than a K7S5A. There is one other option, and that is called nForce.
nForce is a radic new chipset, it features the best integrated Video and Sound ever. It features a GeForce2 MX based video card, and sound that rivals SB Audigy. I have heard that people have not had a ton of issues with the first nForce board (MSI K7N420 Pro), with the biggest issue being getting the Sound to work with 4.1 and 5.1 speakers, but nothing like you'll hear about people using VIA KT266A and K7S5A. nForce could be a very attractive option if u don't have a good video card right now because, at $160-170, K7N420 Pro basically gives you decent Video and Sound for not much more than a Asus A7V266-E. I have not used nForce personally, but I think that it has a lot of potential and it might be perfect for you because you've probably got an older system, and K7N420 Pro would be a better overall upgrade. So, I would really consider nForce if you have the cash. It sounds great, and the issues with the Sound will improve with new drivers.
So, assuming I'm correct in assuming that you're using an old computer right now (pre-K6-2) I would highly recommend K7N420 Pro. So, here's what I would get:
MSI K7N420 Pro motherboard $170 (is heading down)
Athlon-C 1.0GHz $75
Dunno about what CPU Cooler to go for, an Athlon 1.0 doesn't need a Thermalright SK6, so Id look at some cooler comparisions and make a decision. Probably could get a good enough one for $15-25
2 sticks of 128MB of Crucial PC2100 DDR (nForce needs 2 sticks of DDR for maximum video card performance) for $40 if you can hang in there, and wait for prices to come back down again. Right now, it'd be $80.
An Antec, Enermax, or Enlight 350W ATX Case, anywhere from $50-100
Total:$400
Ok, so I went a bit over budget. The biggest suggestion I would make (I need more info about your current system though) is maybe go down to a Duron 800 ($30+$10 for a Volcano2 Heatsink/Fan). That would save u about $50 or so to keep in budget. So, if u have any more q's plz ask.