You'll want the Athlon for your needs, the Sempron is more geared towards business use and basic home PC owner needs like word proccessing, internet browsing, and other low demanding applications like that. Although it will run high end applications such as video and image editing and games, you will see a much larger performance increase with the Athlon in said applications.
The cache is like memory that is built into the proccesor, and is used during memory refresh periods and before the proccesor has to go to the memory to access data. Basically, the cache is the fastest way a proccesor can access information, and with a larger cache, it can proccess more information without having to go to the memory and hard drive.
I would go with the Athlon 2500+ over the Sempron any day, unless a budget plays a factor here.
Intel C2D E6320 / AMD Athlon X2 3800+
Gigabyte 965P DS3 / DFI nF4 Ultra-D
2GB OCZ Gold PC2-6400 / 2GB OCZ Gold PC4000
eVGA 8800GTS 320MB / eVGA 6800GS 256MB
150GB Raptor / 74GB Raptor
2x500GB / 320GB
OCZ GameXStreme 850w / OCZ StealthXStream 600w