What is your current video card?
Is that motherboard combo with a socket A or 754 Sempron?
The Mobile XP 2600+ is a 2.0GHz Barton (512KB L2 cache) with 133MHz FSB. The socket A Sempron 3000+ is a 2.0GHz T-bread (256KB L2 cache) with 166MHz FSB. They should have very similar performance. If it's a socket 754 Sempron then it's a different story, as the K8 processors are very good for gaming.
I don't know what Fry's will offer you but at newegg you can buy a socket 754 mobo for $50 and a Sempron 2800+ for $90. If you overclock it may be worth spending a few bucks more on a board with working AGP/PCI locks, like the EPoX EP-8KDA3I, and if you want future-proof you may consider the ASRock K8 Combo-Z.