I once have my system setup just like that a primary SATA Hard Drive and slave my IDE (7200rpm) both btw. In my case i notice some decrease and problems sometimes BUT thats in my case, anyway what i did was i buy a external enclosure and make the ide hard drive external via USB and its works great but thats just me.
I hope this help you and clear you out some of you doubts
BTW which Hard Drive are you considering into buying??
Check this out:
This one looks good but the only downside is that the cache is only 8mb i would recommend a 16mb cache.
This one i like it in my opinion, it have the 16mb cache .