I don't think the 1.7ghz xp 2100 would make much difference but for $10 it might be worth a try anyway.
If you're not happy with the performance after the upgrade you can start thinking about a MB/CPU/RAM upgrade.
How about surfing over to the HP support forums and searching out what other HPs owners with similar make/model systems have used for upgrades? You should be an idea about other considerations, like a newer PSU required, proprietary motherboards, etc.