I am not sure what others are saying, but you can go up to e8600. The Inspiron 530 has two variants of motherboards, both made by Foxconn and based on the G33M series. The
G33M02 which supports up to Core 2 Duo processors (the e8600 is the fastest compatible cpu)and the DG33M03, which supports up to Core 2 Quad processors.
Inspiron 530 systems sold with Pentium Dual-Core or Core 2 Duo processors can not be upgraded to Core 2 Quad processors without adding at least a 350W power supply and swapping motherboards to the DG33M03 motherboard.
The reason the G33M02 doesn't support Core 2 Quad processors is because it only has 6 voltage regulator ICs, the DG33M03 has 11. This came from
http://en.wikipedia.org/wiki/Dell_Inspiron#Inspiron_530
The same source is saying that you have DDR2 SDRAM at 800 MHz, which can be upgraded up to 8 GB (the 1.0.18 bios update is required).
This is the list of officially supported processors for you
Intel Celeron processor 430
Intel Celeron Processor 420 (1.6 GHz, 800 FSB)
Wolfdale Dual Core E7300, 2.66, 3MB, 65W, M0
Wolfdale Dual Core E8190, 2.66G, 6MB, 1333FSB, 65Watt, C0
Wolfdale Dual Core E8200, 2.66G, 6MB, 1333FSB, 65W, C0
Wolfdale Dual Core E8300, 2.83G, 6MB, 1333FSB, 65W, C0
Wolfdale Dual Core E8400, 3.00G, 6MB, 1333FSB, 65W, C0
Wolfdale Dual Core E8400, 3.00G, 6MB, 1333FSB, 65W, E0
Wolfdale Dual Core E8500, 3.16G, 6MB, 1333FSB, 65W, C0
Wolfdale Dual Core E8500, 3.16G, 6MB, 1333FSB, 65W, E0
Wolfdale Core 2 Duo, E8600, 3.33GHz, 6MB, 1333FSB, 65W, E0
Conroe E1400, 2.00G, 512KB,CCO,M0
Conroe E2140, 1.60G, 1MB, 800FSB, M0
Conroe E2160, 1.80G, 1MB, 800FSB, L2
Conroe E2160, 1.80G, 1MB, 800FSB, M0
Conroe E2180, 2.00G, 1MB, 800FSB, M0
Conroe E2200, 2.20G, 1MB, 800FSB, M0
Conroe E2220, 2.40G, 1MB, PCO, M0
Conroe E4300, 1.80G, 2MB, 800FSB, L2
Conroe E4400, 2.00G, 2MB, 800FSB, L2
Conroe E4500, 2.20G, 2MB, 800FSB, M0
Conroe E4600, 2.40G, 2MB, 800FSB, M0
Conroe E4700, 2.60G, 2MB, 800FSB, G0
Conroe E6320, 1.86G, 4MB, 1066FSB, B2
Conroe E6420, 2.13G, 4MB, 1066FSB, B2
Conroe E6550, 2.33G, 4MB, 1333FSB
Conroe E6600, 2.40G, 4MB, 1066FSB, B2
Conroe E6700, 2.66G, 4MB, 1066FSB, B2
Conroe E6750, 2.66G, 4MB, 1333FSB, G0
Conroe E6850, 3.00G, 4MB, 1333FSB, G0
Conroe E7200, 2.53G, 3MB, WFD, 65W
Intel Pentium Dual-Core E5200(2.5 GHz, 800 MHz FSB)
Intel Core 2 Duo E7400 (2.80 GHz, 1066 MHz FSB)
Intel Core 2 Duo Processor E7600 (3M Cache, 3.06 GHz, 1066 MHz FSB)
Prior to installing the processor, check that you have BIOS Version 1.0.16 installed, when support for new Intel CPU's was added.
This is the latest BIOS: version 1.0.18, Date added 3/11/2009
http://ftp.dell.com/bios/530_1018.EXE
If you have 4 RAM SLOTS you can do 8 Gigs
In case if someone doubts 8 GB RAM on G33M02 board, I found very useful post, here it is link first, but if you trust copy and paste read carefully,
http://en.community.dell.com/support-forums/desktop/f/3514/p/19287044/19527076.aspx#19527076
The post
"Well after reading all the information I could find (in this forum and in other forums) i decided to try the memory upgrade to 8 GB. I bought 4 RAM sticks of 2 GB and changed the RAM. Turned the computer on and started beeping. Turned off and checked all RAM sticks. They all seemed properly connected, but decided to reconnect again. Turned on and still beeping. Took 2 sticks off (leaving only 4 GB) making sure the remaining 2 sticks were connected, turned on and still beeping. Changed the 8 GB RAM back to original 2 GB RAM, turned on and machine worked fine.
Here decided to test the sticks individually. Took out again the original 2 GB RAM sticks and put only one 2 GB RAM stick. Turned on and machine worked and I could get into BIOS (and see there the 2 GB RAM). Took that stick and put another 2 GB RAM stick and again successful. Continued that process with all the 2 GB RAM sticks and all worked fine.
Decided then to try this. Put one 2 GB RAM and turned on PC. Turned it on and it worked (as expected from previous test). Then added a second 2 GB RAM stick and turned on. All worked well, machine turned on, and I could see in the BIOS the full 4 GB RAM installed. Turned off again, added a third 2 GB RAM stick and turned on again. Again successful, I could again see all installed ram (6 GB) in the BIOS. Turned off and added the last 2 GB RAM stick and turned on again. Again, I could see successfully all installed RAM (8 GB RAM) in the machine.
Have tested turning on and off in several occasions, and so far all test the machine keep turning on fine and showing the 8 GB RAM installed on the machine.
So far I can say from my tests that a Dell Inspiron 530 (the plain model, not a 530a/b/c/d model), with a G33M02 motherboard, and the latest BIOS installed (1.0.18) can be upgraded to 8 GB RAM with 4 sticks of 2 GB RAM. Memory I used was Corsair DDR2-667 (VS2GB667D2 G).
Now I will test the installation and running of the 64 bit OS. I will post results after that test, but I do expect that I will run fine if the BIOS is reading correctly the 8 GB RAM installed."
You can install up to Win7 64 bit on this machine, remember only 64 bit can utilize 8 GB of RAM.
Now a little about video card, since it is easy to suggest topic here (compare to processors, he he). If you upgrade your power supply, which is standard ATX, you can go with any video card, 7750 is the newest card which can work on you Ispiron without power supply upgrade.
Now, a little about overclocking DEll.
Only processors with unlocked multipliers can be overclocked in cheap Dell PCs. You have no BIOS control here, only software overclock! I tested this on Inspiron 570 with AMD Phenom II X4 965 Black Edition (unlocked multipliers) by overclocking it from 3.4 GHz stock to 4 GHz with low voltage.
If I missed something please let me know, as it was huge post.
Any comments are welcome.