Normally, the XP2800 would have a larger cache than the Sempron, but it seems the 3000+ has a 512kB cache. Now, you have to realize that an XP2800 doesn't translate into the same thing as a Sempron 2800+. The XP2800 runs @ 2.08GHz, the Sempron 3000+ @ 2.0GHz, both with 512kB L2 cache. Going with Newegg's pricing, you can snag the OEM XP2800+(meaning just the CPU itself) for $115, and the retail version for $138. The retail box Sempron runs for $132, with no OEM chip at Newegg. Personally, I'd get the OEM XP2800+ just because it's a little faster. Do note that there are two versions of the 2800+, one with a 333MHz FSB and one with a 266MHz FSB. The 266MHz FSB is a few dollars more (currently $119 @ Newegg), and is available for those who are limited to a 266MHz FSB max motherboard.
Your motherboard should work fine with either option. The only problem I can see is that the BIOS could be old enough (and this depends on when you got it), that it won't recognize the Sempron. It should boot without problem, however, but it'll tell you something along the lines that you have an "unknow CPU", and you may have to set the multiplier and FSB correctly. More than likely, the motherboard should autodetect the settings. If it doesn't, it'll let you know that you need to correctly set the speed.