To quickly speak on scratch disks, you can set one up in Photoshop CS5.5. Unless they've removed it extremely recently, it should still be there.
As far as a video card, it depends on what are you looking for. Usually NVIDIA Quadro or AMD FireGL cards are tailored for use with things like Adobe products, but those are not cheap parts.
What I would look for is a card that is listed as compatible with Adobe Premiere. While I know you are not working with video, I am very mindful on this point as my observation of Adobe regarding CS5.5 is that they are somewhat finicky about what video parts are compatible with their products. Maybe less so with Photoshop, but I would suggest using a card that is listed by Adobe as compatible with Premiere, as it will certainly be compatible with Photoshop and other CS package software, and also a part that is rated for a more demanding application like Premiere should be more than adequate for your needs with Photoshop. A list of cards that will certainly be friendly to Adobe products at the present time is below.
http://www.adobe.com/products/premiere/tech-specs.html
I don't know if you have any preferences to NVIDIA or AMD, so I will list my possibilities for you to consider among consumer-level cards from that list.
On the AMD side, I would recommend the
AMD Radeon HD 6670. It is about $100 at the moment on Newegg, and offers 1GB of video RAM, which is what Adobe recommends for Photoshop. It scores very nicely on power consumption as well in Anandtech's 2012 benchmarks;
here's consumption under a normal load (a game),
consumption under a OCCT stress test,
and idle power draw. In these cases it performs farily well. Fan noise of the card
is not on this list for some reason, without further information it's probably between the HD 5770 and the HD 7770 on that list. As far as performance, I do not see a "Retouch Artists" test where we could directly measure PS performance but I would suggest it would be about "middle of the road" for your kind of application. If you are willing to spend $20-40 more
the HD 7770 would also be a good pick as it would likely have somewhat better compute performance with similar thermals, noise and power characteristics.
Now NVIDIA is a little more established with Adobe than AMD as using CUDA with CS programs has been possible longer than AMD with OpenCL. OpenCL performs well though, and I am considering the "red" path next time I do a video upgrade if their next few releases can compete with NVIDIA when it comes to Adobe products as they have recently. However presently I am using a NVIDIA GTX 580 for my copy of CS5.5 and have been pleased with it for both Photoshop and Premiere but unfortunately those cards are 2 generations old and are thus largely to be found on the aftermarket these days. For a new purchase - excluding the GTX Titan due to it's cost, and also seeing that Adobe has not vetted the GeForce 700 series yet due to its "youth" - I would suggest a
GTX 680 at the present time if you wish to use a NVIDIA product. Since the new GTX 770 is almost the same as the GTX 680 internally, I think that card and the GTX 780 will be whitelisted as compatible as well, but I hesitate to recommend something not on Adobe's whitelist at this moment. Furthermore, since the GTX 770 is so similar and is lower priced than the GTX 680, there are rebates and other deals being applied to this card presently to make it attractive to buyers.
Please take your time to compare cards on the Anand Bench, and compare scores in both the 2012 and 2013 section the benchmarking section to make your final determination. I personally would think the HD 7770 would suit your particular needs, but you may find some other variable that attracts you to a different card that I have not suggested.