Hi, new member here.
I managed to get abit of an upgrade from my old crappy PC (600MHz Athlon,384RAM) to a compaq pressario S6150UK (Celeron 2.7GHz, 256MB DDR) and it is faster but not as much as i'd like, I know the Celeron isn't that fast due to it being a cheaper cpu than P4 with only small amount of cache but I expected it to be faster.
Will adding another 256MB or 512MB of DDR RAM increase performance?
I sometimes use various applications at once and I also want to put photoshop on it.
Please can anyone help?

In a word, yes. The minimum amount installed should be 512, though nowadays 1GB is not uncommon. The more you multitask or use Photoshop, the more memory you should have. I recommend buying as much as you can afford or as much as your system will take.

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Make sure you match the RAM speed to your FSB speed. Also keep an eye on the timing. The CAS latency is the most important one.

I understand getting same FSB speed but I dont know what you mean by timing and CAS latency??

The CAS (column address select) latency is 1 of four timings that are associated with a memory module. (Actually there are a bunch more, but there are 4 important ones). The CAS latency is the one that is (usually) written as a decimal. You want it to be as low as possible. If I were you I would replace all the RAM with a single stick that has a decent CAS latency. Unless your motherboard supports dual channel RAM, one big memory module will outperform 2 smaller ones.

Much appreciated I will try that, I may get back to you though for more advice!!!
Thanks again

More RAM will definitly do the trick. for best preformence you want to go with one 1GB stick.