hi sam
the dell inspiron has a maximum memory capacity of 4gb ,and supports PC2-5300 ddr2 memory
the dell inspiron 530 has 4 x dimm slots and if you have the 2 spare slots then its just a case of using them,however if you have filled the 2g by using 4 x 512mb then you will need to sacrifice these modules and buy 2 x 2gb modules,this will be the most expensive method and you will have 4 512mb modules spare,however you could allways try and get some money back by selling them on ebay
for a good and cheap memory selection go to http://www.aria.co.uk/Products/Components/Memory/DDR2/DDR2+667+%28PC5300%2F5400%29/?p=cF9zdHlsZT1saXN0JnBfcHJvZHVjdHNQZXJQYWdlPSY=,aria have the best selection for memory in the u.k
for example for a OCZ 4GB PC2-5400 memory upgrade(http://www.aria.co.uk/Products/Components/Memory/DDR2/DDR2+667+%28PC5300%2F5400%29/OCZ+4GB+PC2-5400+Vista+Upgrade+%282x2GB%29+?productId=26604) you can buy this for just £52.82 inc. VAT and for ocz this is extremely cheap
ocz are probally the best memory manufactuer apart form kingston and corsair,and their quality is obvious with stylish heatsinks and low latency timings
also the Arianet 2GB PC2-5300 DDR2 SDRAM (http://www.aria.co.uk/Products/Components/Memory/DDR2/DDR2+667+%28PC5300%2F5400%29/Arianet+2GB+PC2-5300+DDR2+SDRAM+?productId=30175)is a good deal at just under£20,and for £40 for 4gb this is also worth considering
finally i am using these items as examples however i would recommend you visit http://www.crucial.com/ and run the online memory utility
this will scan your p.c and give you a report on what memory your p.c needs and also you can buy diretly from crucial themselves,however crucial tend to be quite expensive when compared to aria and other websites
also try my local p.c shop at http://www.moreton-tech.co.uk/shopdisplayproducts.asp?id=21&cat=DDR2+Memory,speak to steve and remember to say i recommended them,i buy my memory from there and compared to other online retailers they are quite competetive
any problems let me know
good luck sam !