Ok,
Of course Dell doesn't list the power supply in your system configuration. But according to your exact specs, I would imagine they put in a 300 watt power supply.
It is hard to tell what memory you have installed. I know you have 400mhz and it seems to be DDR2 according to the manual. But that is a pretty rare breed of memory. But according to the manual, it is DDR2 400.
You do have 2 x 512 sticks of memory and two slots available. Another 2 x 512 would be a wise choice for current games. I don't know any good UK resources, but at Newegg.com, they have a 2 x 512 DDR2 400 pair for only $24.99 USD. That is a worthwhile upgrade for sure.
The video card I would recommend (and I have been a lot lately

) is the Nvidia 8800 GT. It is a really good bang per buck card.
You're looking at spending around $180 USD for the 8800 GT. Here is a link to one I recommend -
Newegg.com - EVGA 512-P3-N801-AR GeForce 8800 GT 512MB 256-bit GDDR3 PCI Express 2.0 x16 HDCP Ready SLI Supported Video Card - Desktop Graphics / Video Cards
For a little bit more money, you can go with an 8800 GTS. Make sure you go with the 512 megabyte G92 edition if you do. You're looking at spending around $210 USD for the 8800 GTS. In most reviews it's not a major difference above the 8800 gt so you may want to save a few bucks and stick with the GT. Here is the one I recommend -
Newegg.com - PNY VCG88GTS5XPB GeForce 8800GTS (G92) 512MB 256-bit GDDR3 PCI Express 2.0 x16 HDCP Ready SLI Supported Video Card - Desktop Graphics / Video Cards
To go up from there, the next card up that's good bang per buck would be the 9800 GTX. That's going to run you around $280 USD. Sounds a bit out of your range...especially that you'll have to go with a new power supply.
You'll notice in this review at lower resolutions, there isn't a major difference in most of the games between the GTS and GT. The 9800 GTX is clearly ahead of them. But you'll get an idea of what performance difference to expect when you pay more money -
Nvidia GeForce 9800 GTX Review : Introduction - Tom's Hardware
WIth any of those 3 video cards, you will need to upgrade your power supply unit to a 500 watt or better, and you will need to get one with 6 pin PCI - E connectors.
If any of those with a power supply are sounding too expensive for you, then going down from there would be the 8600 GTS. This is the card I would recommend -
Newegg.com - EVGA 256-P2-N761-AR GeForce 8600 GTS 256MB 128-bit GDDR3 PCI Express x16 HDCP Ready SLI Supported Video Card - Desktop Graphics / Video Cards
That card is $119 USD which is obviously a lot cheaper. Also, that card only requires a 350 watt power supply, so if you can pop your power supply out and find out what the exact power rating is, you may not even have to upgrade your power supply. That'll be a good price saving. But depending on what games you are playing, it may not be a worthwhile upgrade.
Hope that helps!
