Is going from a AMd XP 1800+ to Sempron 2800+ much of an upgrade, especially for games? The fact that its being compared to a Celeron scares me, although I admit that I know VERY little about what makes a chip good or not, completely new to computers.
Get sempron 3100 with a Chaintech Nf350 MB man. CPU is like 100 dollar and Mother board is like $75. it will be 2x or more faster than ur 1800xp. By the way there is a big difference between an sempron 3100 and 2800