With the Abit TH7II board and good memory you should be able to take a 1.7 to 1.9, 1.8 to 2.0, 1.9 to 2.1, and a 2.0 to 2.2 without issue. It all is subject to the quality of the cpu, board, and memory but most people have not had any issue getting a 200mhz bump in overclocking with the P4 at this time. I would buy a socket 478 chip unless you find a socket 423 for a very good deal and then you should buy a Asus P4T motherboard if you plan on overclocking. Asus and Gigabyte will have their s478 boards out shortly but the Abit board is very good if you are overclocking, if not the Intel D850MD/V boards offer excellent performance and stability without overclocking options.
To answer your question, you will not really notice the difference between a 1.8 or 2.0g chip in most applications. The s423 is a dead end solution, s478 is where Intel is headed for the next couple of years. 🙂