In my experience, games these days demand a great deal of videocard based performance. I currently have 8 machines running in my basement (a semi-long term LAN party set up) - all of them running at various speeds, with various setups. Point being - of all my observations during the past few years - the thing that makes for a great gaming experience is a good videocard. Any sort of faster end Athlon processor will do you well today. I'd say go for something around the 90-130 dollar range - like an Athlon XP 2500+ or maybe even up to a 2700+. If you're really tight on budget, or want to keep your FSB speed low, check out an Athlon XP 2000+ - they're fast and rather inexpensive currently. Try to save money on your processor so you can push it all into the videocard. The way card prices seem to run these days - high end models cost way too much. Avoid picking up a Radeon 9800 or GeForce FX 5900 - those will only drop in price by 100 dollars in the next couple of months. I'd suggest something like a Radeon 9700 Pro - its a great card - doesn't have too high of a price tag, and can utilize all sorts of new features like Direct X9 and AGP 8x. If you're still strapped for cash beyond that - go for something lower - maybe a Radeon 9000 Pro or a GeForce 4 Ti4200 (8x AGP, 128 MB RAM) - these cards are good and yet don't cost too much cash - (just above 100 dollars). I'd strongly suggest avoiding the GeForce4 MX card series - they've given me nothing but trouble and bad performance.
And for a last note - on the hard drive - large amounts of storage for a low price... sounds like a good idea, but there's more than that. If you want your computer to run quickly with hard drive operations you'll want to make sure your drive is at least a 7200 read speed.
Keep in mind that the Asus board you have in mind is a KT400 chipset. This chipset is good for 133 FSB operations, but only fair for 166 FSB work. This means that the fastest processor you could strap on before running into inefficiencies (as compaired to an nForce2 Chipset board) would be an Athlon XP 2400+ This processor might be a wise choice for a small budget - as they only cost around 85$ these days. It would also leave you ample room to beef up that all-important-to-gaming videocard.
Good luck! Feel free to check out www.jag85.com, my web site.
-Jeff-