well, lately people have been complaining that asus is going downhill. they were fairly decent. that motherboard however looks like a fairly good one.
well, i sort of expected that would be your motherboard and it looks like we're off to a great start.
that motherboard has the following:
a PCI-express 16x slot. perfect for a good gaming graphics card.
an AM2 socket. perfect for a great dual core amd processor
4x 240 pin pc2 6400 ddr2 800mhz RAM slots. that's standard memory these days.
but that's about all thats good. obviously, it's time to start upgrading. since you didn't really tell me what your budget was, i'm just going to go with some fairly top of the line parts.
graphics card:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814130304
that's an 8800gts 640 mb card. excellent for gaming especially if you want to play something like crysis or bioshock.
however, at this point i would recommend an 8800gt 512mb because it outperforms that card by a good bit. unfortunately they're out of stock in a lot of places. so if you want one, you're on your own tiger.
processor:
http://www.newegg.com/Product/Product.aspx?Item=N82E16819103773
amd 64 x2 athlon 6000+ 3.0 ghz
thats a fantastic processor. and it will fit that AM2 slot perfectly.
memory:
http://www.newegg.com/Product/Product.aspx?Item=N82E16820146565
and there's 2 gigs of memory to compliment the system.
now...that's some very powerful stuff. so you need a good power supply to make sure it'll all run perfectly.
here's a power supply thats fairly cheap and has lots of head room so you can add stuff later and still be able to power the system with confidence:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817371007
i'm not entirely sure how you're doing in terms of cooling for that box. you may want to consider adding a few fans to it in order to keep the whole thing cold enough so your whole thing doesn't over heat and fry itself. and fans are very easy to find.
the brand you want to look at for fans is Zalman. they have some very nice stuff.
again, i assumed there wasn't really a budget. if you DO have to work around a budget, tell me now so i don't force you to overspend or search for some cheaper stuff on your own.
but if you can get those parts, then you ought to be in VERY good gaming shape.