Plus you have to take your power supply into account. If you have the original PSU that went into your rig, then it might not have enough amps on the 12 volt rail to power a 3850. Wait a bit if you can though, they're overdue on the 4450's and 4350's. There might be an AGP version.
Edited to add, I just read your last post. You probably won't have the amps for anything above the 3650 or a 4350 if an AGP version arrives. I'd consider a new barebones system. I've bought MSI barebones before until I could afford better.
Here's one that's very cheap:http://www.newegg.com/Product/Product.aspx?Item=N82E16856167035
Here's the motherboard CPU support list:http://global.msi.com.tw/index.php?func=prodcpu2&prod_no=1572&maincat_no=1