dont listen to them the 5900 is a decent card by todays standerds. before i got my sli rig and my 6800's i was useing 5900's and with a little oc'ing i was able to play doom 3 @ 1024 with every thing on the highest detail with 4x aa and AF and still be pulling of 40+ ps and around 30ish for big fire fights. granted i was ocing the card so that it was giveing a 9800 xt a run for its money. dispite what is said with a little ocing you will have no problems with the games that are out today.
as for the semperon they arnt like the celeron. they were designed as budget cpu's and to continue the socket a (almost dead now) and the socket 754 while the 64bit chips ran on the 939 socket. the semperon is alot faster then the celeron and isnt fair to compair it with one. the cpu will run about in par with the intell 2.6ghz if not the 2.8ghz.
as for the ECS mobo well its a budget pc so you cant expect the world of it...
that said looks fine to me