It all depends on what you do with your PC. If all you do is surf the web and use an office suite, the current configuration that you have is enough. If you are into games or heavy CPU intensive applications, the upgrade will be well worth it.
According to a currency converter I used, R800 equals around $120.
Since you canít really afford the 2.8 P4, I would recommend going with an AMD Athlon XP CPU and board. You can get a 2800+ and a decent board for around $150. That it almost a cheap as buying a 2.8 P4
You money may be better spent on adding an additional 512MB of RAM for $40-$50 and adding a 7,200 RPM hard drive. Those are some of the cheapest/smartest ways of getting more performance for your computer. I donít think that a 2.8 Celeron is much better if any better than a 1.7 P4.
You can also upgrade your CPU with a Celeron D processor. They are a lot better than the regular Celerons. They are also around the same price. You can get a 2.8 Celeron D for around $91.
Hereís the list of compatible processors that will work with your board.