Mobo: ASUS P8H61M-C (109)
PSU: SUPERFLOWER HX 600M (99)
Case: DEEPCOOL TESSERACT (65)
OS: WINDOWS 7 HOME PREM (120)
Thermal Paste: ARCTIC COOLING MX-4 (18)
Total cost: $411
Buy at PC Themes, #04-15, and ask for discounts. DIY yourself. If you can't into DIY, get PC Themes to assemble for you for a fee.
Notes:
1) This list of parts is intended to allow you to get a rig that allows you to use your GT 730 with minimal cost and aims to make maximum use of the parts in your HP rig. The rig is also intended for maximum upgradability in the distant future.
2) Things to reuse: DVD-RW, HDD, RAM, CPU, GPU
3) The Asus P8H61M-C is compatible with your i5-2320 and allows you to reuse your CPU. However, some staring at the mobo picture at the HP site has led me to believe that your old mobo actually conforms to standard M-ATX specs and thus is actually compatible with the new case. Thus, it may be a good idea to hold back the purchase of the mobo, the OS and the thermal paste until it has been proven that your mobo is incompatible with the new casing.
4) The Deepcool Tesseract comes with 4X 3.5'' HDD bays for you to be able to put in both your new and old HDDs into the same case.
5) Not to say bad things about your purchases, but the GT 730 is not really what I would consider a good value GPU upgrade. It can play DOTA and CS: GO at max settings in 1080p, but you are advised to play Archage in Low settings in 1080p as Archage is a very GPU demanding game. (I hoped that you had not brought the 4GB version of the GT 730, though.)
6) Do sound off if you wish to have more upgrades other than these parts.