Since parts have been purchased, and you are on a budget, try the adapter and existing hard drive or see if you can get a bigger hard drive. 78GB might last a while, but then again, who knows how long hard drive prices will be high and you could end up using that space and needing to buy a larger (SATA) drive anyway and have to rebuild the system.
Biggest advice I have after building dozens of machines over the years? Don't skimp on RAM. You don't need to have the fastest RAM, but lots of it never hurts. Since you are on a budget, I would shoot for 4 GB. Others will say 8GB, but I do some pretty extensive gaming, Photoshopping, and even video editing and do real well on 4GB in my 3 year old system. Granted, I went pretty high end on components back then, thus the reason I've gotten along well with it for so long.
