Review
Heat is not an issue since A64 processors are rated to run at 80*C or something around that. Every time any user mentions heat issues, they should only be concerned with that if they are overclocking or the system is overheating due to poor airflow/heatsink+fan. Otherwise either system that runs at 10*C or 60*C is well off. There haven't been any detailed tests that show a cpu having a longer life running at lower temperatures that is actually applicable for the common user. Either cpu will last 10 years.
Get A64 3000+ 90nm for $140 from Monarch Computer:
ensures highest possible overclocking potential between Sempron and A64 2800+
ensures best upgrade path due to socket 939
ensures better performance than either of the chips for not much more.