Personally i wouldnt't reccomend using a company. What you want to do is build your own or if you know anyone that would do it for you, have them do it because you will get the best performance for the best money and you know exactly what is inside that case and exactly what is on on the hard drive when you first get it - preferably nothing!!
If you live anywhere near me i would build you the system you want for cost of components and £25 for labour but the chances of that are pretty slim.
As for the components you want, the processor is good so long as it is coupled with a good motherboard. The best by far in my opinion would be ABIT - you get top performance, all the bundled extras you need (cables, USB expansion brackets) and everything else. The manuals are written well and Abit BIOS's are the best availiable.
As with your RAM, 1Gb will definatly be enough. I play a lot of games (Flight Simulator 2004 mainly which is VERY demanding on resources) and i have only 512Mb and it is ok. 1Gb should mean you wont need to upgrade it, as long as you have 2 512Mb sticks and run it in "Dual Channel Mode" to take advantage of it.
Your HDD should be fine, just make sure that you have SATA ones with a 7200rpm spin speed and 8mb of cache. This is the fastest option availiable unless you want to spend lots on a 10,000rpm Western Digital Raptor which is the fastest SATA hard drive out at the moment. You could also, instead of buying a 120Gb drive, buy 2 seperate SATA 80Gb drives and run them in a RAID (0) array. I am not sure if you are familiar with this but basically it is where the 2 drives share bits of data and while one drive is reading, the other can que up the next bit of file so it is a great enhancement in speed all round - upwards of 30% i think.
Your graphics card is pretty good - the same as mine but alas, its getting on. If you want your machine to be effectivly futureproof, your only option really is to shell out on a Radeon X800 or FX6800 - the new generation cards. They are about double the price of a 9800pro coming in at just under £300 each. The performance increase is m a s s i v e however, look at the picture attached. The 9800 pro strains to run Flight Simulator 2004 at full settings and that is a game released last year so i can guarantee you will have problems with it running new stuff in the future.
You can pick up DVD burners quite cheap now, for around £60 you can get a new generation one that supports dual layer burning - they are able to write to dual layer discs that have massive capacities - nearly 9Gb instead of your normal 4.7Gb
With a system like that you would definatly want a good PSU - for a system like the one you would like, i would not reccomend using a generic PSU at all - it would put too much strain on it. You want one from a manufacturer such as Antec, Globalwin, Zalman, Akasa... Any of the well known brands.
As i said before, always try to stay away from companies, they do cut corners as any member of this forum will tell you.
Hope that explains the majority of things,
Regards,
Chris
P.S Reefsmoka - i have seen DDR2 Benchmarks, they suck. This is because they are very high latency - i think at the mo the best is CAS4 - was reading about the new intel LGA775 which must have DDR2 memory. Its not wise to buy one of them at the mo!
. As for the ram, a gig is plenty, you shouldnt need to upgrade for a few years either, but if you do it should be alot cheaper because DDR 2 is lurking round the main stream computer corner 
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) could I trust that the motherboard and other components would be half decent (I'm thinking Mesh or Evesham), or do I need to insist on particular makes?
