Newegg.com - MSI 785GTM-E45 AM3/AM2+/AM2 AMD 785G HDMI Micro ATX AMD Motherboard
I recommend looking into this and other MSI mobo's, so far I am second one and I have not had any major issues at all so far.
The reason why I recommended this because this mobo has ati hd 4200 integrated which should handle the sims game pretty well.
I am also sure that HP case can have the I/O panel knocked out, look at the Back of the pc and make sure the I/O is silver then, and flimsy, if it is your good to go.
I do recommend you look at the size of the ATX PSU before buying the wrong one and size.
This will only uitilize DDR2 800 ram up to atleast 2-4gb.
Also if you can swing it look at the Athlon ii X4 630 quadcore, you would benefit better from that and doing multiple medium size or heavy tasks programs, it will eat it alive for breakfest and still want seconds...