Almost any modern graphics card will give you a major boost in gaming. Read this article and get what you feel you can afford:
http://www.tomshardware.com/reviews/radeon-geforce-price,2323.html
I would avoid the issues with sli/crossfire, and look at a single good card.
Check out how many pcie power connectors the graphics card you selected requires. For anything more than a modest card, it will require a 6 pin pci-e connector, and possibly a second 6 or 8 pin pcie psu connector.
PSU needs for a gaming pc are driven by the graphics configuration. Look for a QUALITY psu. Something in tier 2 on this list:
http://www.eggxpert.com/forums/thread/323050.aspx
Select one that has at least the requisite number of pci-e connectors. A quality psu will be able to deliver the amperage required by those connectors.
A psu is the last place to go cheap. No end of strange problems and failures come with a marginal psu.
For any ram you are considering, do your own homework.
Go to the ram vendor's web site, and access their configurator.
Corsair, Kingston, Patriot, OCZ and others have them.
Their compatibility list is more current than the motherboard vendor's QVL lists which rarely get updated.
Enter your mobo or PC, and get a list of compatible ram sticks.
Here are a few links:
http://www.crucial.com/index.aspx
http://www.corsair.com/configurator/default.aspx
http://kingston.com/
http://conf.ocztechnology.com/index.php?c=1
Cpu performance is not very sensitive to ram speeds.
If you look at real application and game benchmarks(vs. synthetic tests),
you will see negligible difference in performance between the slowest DDR2 and the fastest DDR3 ram.
Perhaps 1-2%. Not worth it to me.
Don't pay extra for faster ram or better timings unless you are a maximum overclocker.
There is generally no real world(vs. synthetic transfer rate benchmarks) performance advantage to raid of any kind.
Go to www.storagereview.com at this link:
http://faq.storagereview.com/tiki-index.php?page=SingleDriveVsRaid0
There are some specific applications that will benefit, but
gaming is not one of them.
---good luck---