<< firstly I know it badly needs an SSD, for which I'm thinking of the Samsung 250gb 840 EVO series >>
This 250GB Samsung 840 Evo at NZ$285.89 will bite off a big chunk of your budget so I'd suggest get the 120GB at NZ$168.94 instead. Reinstall your OS on this SSD but put the games & datas on your 1.5TB Hdd.
<< and my video card is struggling with the latest releases. >>
You'll need a gx card like GTX660 at NZ$342.61 if you want vast performance improvement over Hd5850, these 2 items together exceeds your budget by $11.55 already.
<< The ram could do with another 4GB >>
You didn't say what version of your OS was but if it's a 32bit then anything more than 3.5GB of Ram is a waste.