No semprons are not fine for gaming.
I have a sempron 2600+, 512MB Ram, Radeon 9550.
I run Doom 3 at 640x480 with every setting to low at 34 fps average.
My friend with an Athlon 2800+, 512MB ram, Radeon 9200SE
runs Doom 3 at 1024x768 with everything on low at 40 fps.