the big problem you are going to experience with your Dell Dimension 3000 is the lack of pci-express or agp slots,this is unusual with modern motherboards however for some reason dell have some systems that come without these making graphic card upgrades a bit difficult
however the good news for you is there is still some pci only graphic cards and with windows 7(and vista) requiring a card that supports pixel and shader model 2.0 and theres some pci cards that support this compliance
one such card is the PNY Verto GeForce6 6200 256MB( http://www.ebuyer.com/product/121534
) ...this supports pixel and shader model 3.0 and has 256mb of ddr2 ram.however this card wont run the latest games but should run older games such as doom3,quake 4 and max payne
but most importantly it will run the aero interface in windows 7 and costing just £36.38inc vat this represents excellent value
however when you buy a graphic card you want to be able to run some of the latest games and allthough this isnt gonna be easy with a pci card its not impossible,and there are some better spec cards avalible
one of these cards include the "PNY GeForce 9400GT 1024MB"( http://www.product-reviews.net/review/20090625/pny-geforce-9400gt-1024mb-pci-graphics-card/
) ...this is a pretty good card for a pci card and also supports low profile fitting so if you have a smaller case this is ideal
however the specs for this card are not that bad,these include a 550mhz core clock and a mammoth 1gb of ddr3 memory clocked at 800mhz,,,and this will run aero 3d aswell as some of the latest games abit at lower detail levels
so with aero 3d and gaming support you may also need multimedia features and the "VisionTek Radeon HD 2400 Pro 256MB DDR2 PCI"( http://www.pricescan.com/VisionTek-Radeon-HD-2400-Pro-256MB-DDR2-PCI/co/229185.html
) has all these boxes ticked and does a sterling job in all area,s
it supports pixel and shader model 4.1 and direct x 10...it will run aero 3d and run the latest games( again maybe at lower detail ) however this also supports ATI Avivo HD technology and will provide hardware based H.264/VC-1 decoding of Blu-ray and HD DVD movies
so out of all these cards the Radeon HD 2400 Pro looks the best bet,,,it may be a small card but as pci cards go its one of the best....
note : some people ask me why there are no powerful pci based cards like the geforce 9600gt and radeon HD4860,,,well its all about bandwidth,,,,the pci bus can only run at 133mb/sec were as the pci-express bus runs at 4gb/sec with the latest pci-e v2.0 running at 8.0gb/sec...thats a massive 55 times faster than the standard pci slot your using !
so if any manufactuer tried to make a gtx260 in the pci bus it simply wouldnt work...however since the cards i have recommended require less power and are not that fast they work well using the pci bus
recommendations : go with either the geforce 9400gt or radeon HD2400pro...the geforce 6200 is old hat and nowhere near the performance of the others,and they all cost around the same
theres also the geforce fx5200 ( http://www.ebuyer.com/product/119724
) however this is also a old card and has inferior speeds,however if its just aero 3d you want and no gaming capabilities then the fx5200 would be fine,and at just £50.59inc vat its not too bad
i hope this has helped,any problems let me know
good luck zack !