I have an old GeForce 7800 and a 500w power supply (not sure on make and model as this is for someone else, will get this info later) that I'd like to reuse if feasible. My main questions are wether or not a 500w PS will be sufficient, wether or not I would notice a huge difference in Photoshop with a 5770 vs the old 7800, and also any opinions on a SSD for Win 7 and PS to be installed on.
You can get by with any modern IGP for CS4 PS. 7800 is overkill, but since you got it, reuse it. There are 3 variants: GS, GT & GTX, from slowest to fastest. A 350W would be the minimum for GTX. Higher if the PSU is a generic no-name. So your 500W is ok. But I would not gamble my $1000 parts on a generic. What's the make & model of the psu?
The SSD is a wrong choice for stills cuz you will be storing your stills on a HDD. SSD is great for OS and games/apps, not data. If you can afford it, get it + a Samsung F3 500GB/1TB for backup/data. RAW is frigging huge, and the fastest way to read/write the files would be HDD.