First, welcome to the forums
I suspect the answer is not very well at all. The inetrated graphics are really only suitable for office and general stuff.
Still you might be able to play at the very lowest settings and get something. I might suggest the Jetway board using the ATI chipset (can't remember the name offhand as many people have reported overclocking the graphics part very well such that its much faster than any 6150 board. It also has HD sound, SATA 2 and fast USB performance so it'll be a board that should last you a while - it looks like you'll be looking to keep it for a while. The other reason I'd recommend going for a S939 board is that when S939 becomes the budget option and time passes dual-cores should become very cheap (comparatively) and offer you some a decent upgrade with performance/$.
Also you can overclock your CPU well in that board as you'll be looking for maximum bang per buck. That board can typically do 300mhz for HTT or pretty close - at that speeds the onboard NIC will go if that's an issue but you should still be able to overclock pretty high before that goes (check).
Anyway going for the sempron is a perfectly good choice - currently I'd say it's one of the budget kings (Well maybe next to the Intel D 805) and will compare well enough for a similiar single-core Venice etc (when you upgrade make it to dual-core)
512MB of RAM is going to limit you alot - 1GB is standard for any gaming machine but if you are running very low graphics perhaps you can get away with it - what I will say is be prepared to run a very lean machine eg a user account with next to no background apps or services running bar the key ones. Just when you purchase RAM ensure that it is a single 512MB stick so you can add a second one later.
In fact, given the hype about Conroe and the arrival of AM2 you can expect DDR sticks to be going for very cheap at second hand for sale venues (inc at these forums). In fact even now might be a good oportunity to buy from someone here - you might be able to get 1GB at very good prices.
(edit) I'm not sure S939 have arrived in the retail channels yet - there meant to arrive mid this year. In which case just check second hand places/hot deals/anandtech for & trade for some bargain S939 Venices or so. There should be enough people upgrading to dual-core etc that you can get a very nice price.