I recommend the following components for upgrade:
Duron 1.1 GHz (or) Athlon XP 1700+ (or) 1800+
ASUS or MSI mobo with nForce 415-D Chipset (as you don't need integrated graphics, 415-D is the best bet for you, 220-D and 420-D comes with integrated graphics, you may buy 220-D if you can't get 415-D, and only when 415-D or 220-D are unavailable, go for 420-D)
256 MB DDR RAM from Micron / Crucial (DDR266 is sufficient for Duron/Athlon XP, DDR333 won't give noticeable performance boost)
Special Notes:
(If you want to upgrade only for gaming) Without changing your graphics card, you will get similar gaming performance from 1.1 GHz Duron and Athlon XP 1700+ / 1800+. Athlon XP will boost performance in applications that need better CPU power only.
Some people say that nForce chipsets are more stable than VIA chipsets. Though I personally use MSI K7N 420 Pro (nForce 420-D Chipset), I don't think it is true. If you don't want to buy nForce chipset based mobo, buy MSI KT3 Ultra (KT333 chipset) without RAID. This mobo gives good performance from its price points.
GeForce2 MX400 would allow you to play all games till next year. But if you need better gaming experience for one year, buy a card based on GeForce3 Ti200 chip. It should be at least 3 to 4 times better than GeForce2 MX 400 ( See THG VGA Chart in Graphics Guide). If you are want to pay lesser than GeForce3 Ti200, buy a card based on GeForce4 MX 440 (should be better than GeForce2 MX 400 than 2 to 3 times)
You can know about current prices of the components in www.pricegrabber.com. This site is really cool for knowing price of pc components.
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