Motherboard?,OS,hard drive change,CPU upgrade

In a previous question re: motherboards (answered very well-thanks), I was/am going to upgrade to either an ASUS A8N-SLI Premium or a MSI K8N Diamond Plus (again-I've decided on this-I think).The CPU will be an AMD A64 X2 3800+.Now comes the big question(s)--or at least for me. After talking to MSI tech. support,I was told that I would have to change my hard drive type and re-install the OS (Win XP/Home Ed.) also.According to the tech. info. re: these two mobos,IDE conections(2) exist on the mobo.I certainly am not an expert on PC's but have owned many and upgraded for some 25 years.I would very much appreciate someone trying to explain all this to me and why.Thanks.