We can give better advice if you list the maker and model of your motherboard.
You can use 800MHz RAM alongside 667 RAM, but it will only run at maximum 667MHz. That is, any faster speed RAM you add will slow down to match the existing slower RAM. It should work fine however and give you all the benefits of more RAM - assuming you're running a 64-bit OS.
If you're planning to replace your existing RAM, give the specs for your motherboard for recommendations. If you include the mfg webpage for your board, saves those helping you lots of time!
Since you only have two RAM slots, gotta replace the existing RAM.
The HP spec page gives you the answers you need:
* Dual channel memory architecture
* Two 240-pin DDR2 DIMM sockets
* Supported DIMM types:
o PC2-4200 (533 MHz)
o PC2-5300 (667 MHz)
o PC2-6400 (800 MHz)
* Non-ECC memory only, unbuffered
* Supports 2GB DDR2 DIMMs
* Supports up to 4 GB*
o 32 bit PCs cannot address a full 4.0 GB of memory.
It says max of 800MHz, and max of 2 - 2GBs.
I'd recommend a good brand of PC2-6400 / DDR2 800 RAM with low voltage and low/fast latency, such as one of these:
505090 is correct, that the power supply requirements are indeed a factor with video cards. (Both the amount of power on the 12v rail(s), as well as the power supply's available connectors <molex and/or 6/8 pin PCI-E connectors.)
Further, other questions to consider:
1. Are any expansion boards currently occupying the single PCI-E X1 slot on your MB? (The small slot below the top most of the 4 slots.) Video cards are either 1 or 2 slot cards. If you have hardware in the smallest slot, you will be limited to single slot cards.)
Assumption: No PCI-E connectors (since it is a HP computer), and the power supply is barely adequate for the pc (all OEMs do this). I would figure something in the range of 400w total, with a total +12v output on a single rail of around 15a.
(You should be able to look on the large sticker attached to the power supply to get this information.)
2. It is never a good thing to a buy a video card that is "too powerful" for your current CPU, unless you plan to buy the video card first. (And, then, move the card to a new computer.)
As such, since you didn't mention what cpu you have, I am going to assume you have the best your MB can take, a AMD Athlon 64 X2 4800+ (duel core, runs at 2.5 Ghz).
3. What games do you play?
(Assumption: Everything but the most modern of games, mostly at medium quality settings)
4. What resolution do you play at?
(Assumption: 1680 x 1050 22" widescreen monitor)
Check your Windows display settings for this information. The smaller the total resolution (mulitple the two numbers), the less powerful the graphics card you need.
5. Does your case have room for an ATX power supply replacement? (Meaning, is your computer's power supply "standard sized.")
6. Are you comfortable enough with computers to replace a computer's power supply?
If the answer to question 6 is no, then I would recommend the following:
It doesn't require a separate power connector, SHOULD work with the lower power of your power supply, and will adequately match your processor. It offers multiple video output options, is relatively low priced, and is a respectively powerful card for its kind. (In typical use, it is used in people's home theatre pcs. But, it should work well with your pc's era.)
If the answer to question 6 is yes, and you have the budget to buy a new power supply to, then:
This combo supports DirectX 11 (versus only 10.1), has 2X the performance in most games over the 4670, and it will also be a good match for your cpu. (However, your cpu will probably be a limiting factor in some games before the video card.)
And if you replace your PSU, make sure the new one has the proper connectors. Shouldn't be a problem, you'll likely have far more than you need. But if you get a GPU that wants a power connection, need to have one. The HD4670 or 4650 are good products, and cheap for the performance. Might read over this "Advertorial" by Tom's: http://www.tomshardware.com/reviews/ATI-Radeon-HD4650-HD4670,2444.html
I don't know much about GPUs since I use my built-in graphics and even cut down the 512MBs it steals from my MB to 128MBs!!! Clearly, I'm not a gamer! The last graphics card I bought was a 64MB AGP about 9 years ago!!!