Well your mouse seems modern enough, I think you might need a keyboard upgrade. Any $20 wireless or not modern keyboard would suffice, leaving you with $130 USD. Since prices are jacked up a bit in NZ, I'm supposing you'll have around $160 NZD to spend. With that amount you could probably get a PowerColor PCS HD 4770 if you can stretch your budget (it costs ~ $168 from PC Pacific, but I don't really know if that store's trustworthy, it's half half based on Pricespy reviews). Otherwise, for $149.31 from AlphaCity (great store) you can get a Sapphire Radeon HD 5670, and I suppose you would see a relatively decent performance boost.
Largely depends on what you will be doing, I am going to assume this is for gaming, so it depends on what games you want to run.
A keyboard is a keyboard. If it ain't broke or showing signs of breaking soon stick with what you got. I have no idea as far as the mouse you have, but if it is optical it is likely sufficient for most use, but if you need a new one shop for one you like and fits your budget. Mouse and Keyboard are largely personal preference. I am partial to my Logitech G5, but that would be a little too expensive, but the mx518 is about $40 USD.
Start with a graphics card, if you can afford to get a new mouse after that, find one that feels good to you and is affordable, go for a keyboard last.
Heh, I'm lucky if a mouse lasts through 6 months of Counter Strike, much less 4 years!