You can't really compare the ghz rating if you're comparing two different types of processors. A 3.6ghz Pentium 4 is going to be much slower than a 2.8ghz Core2Duo... but both would be plenty fast enough for FFXI.
Intel is currently the fastest with their quad core Core 2 Duo chips. They're about to release new 45nm chips early next year (probably around April) which will cause current chip prices to drop a lot.
AMD still has some very fast processor offerings, and will run FFXI just as well as an Intel chip will (mostly because the game is so old). I'm currently running a X2 4400+ and I'm able to play the latest and greatest games (also because I have a decent video card, as well).
Either way, if you want a good processor with a decent motherboard that will last you a while, any Core2Duo (dual or quad core) or an AMD 6000+ (which will run around $170) will last you a long time. Just make sure you get a decent motherboard.
I personally don't like going the cheap route with CPU's and motherboards anymore, so my suggestions might be mroe than what you're wanting to spend, as you might also have to invest in a better power supply and DDR2 memory if you're still sporting DDR1 (like me!).
You can usually save a litlte money by going with AMD, for the moment. So here are a couple suggestions.
AMD Athlon 64 X2 6000+ Retail ($159)
ASUS M2N32-SLI Deluxe Wireless Edition ($169)
If you are looking for a video card, you've got a few options, as well. The Geforce 8800 GT video cards just came out and are going for around $250 to $300, depending on how overclcoked it is from the factory. They're the most popular video card right now because they aren't super expensive and they will get you almost the same performance as an 8800 GTX which costs several hundred dollars more. If you don't want to spend that much on a video card, try to find a Geforce 7900 GT or GTX. Since the 8800 GT was released, these have been falling in price quite a bit. You might have to buy a used online, but I've seen them for less than $100 a few times.
It's always good to check hardware forums, and people will usually sell their older stuff they don't need anymore for pretty cheap. I'd recommend
HardOCP's forums. Check the CPU/motherboard, gaming, hot deals, and for sale/trade sections.
http://www.hardforum.com/