The 537s is pretty much as much better as it is more expensive. A 537-non-slim would be an optimal choice. 2x2 GB of RAM instead of 4x1 GB is always good, an add-on graphics card also takes off that load from the RAM, and the CPU is quite much faster.
Just went to the Dell site, and I would really recommend getting the 546 with either Athlon x2 7550 or 7750. 4 GB of RAM is okay, but you could well get by with 3, as you're going to use XP anyways, 0.25GB(ish) is not worth 35 $. After buying, add a 4350 yourself:
http://www.newegg.com/Product/Produc...igabyte%204350 Do you really need a 56k modem? You're most probably operating on an ADSL connection and don't need it for internet at least... Maybe some fax stuff, I wouldn't know.
704 $ (-35$ with 3 GB RAM, -60$ without the 4350 pre-installed, and it's 35 from newegg with free shipping and 25 after MIR):
SYSTEM COLOR Piano Black edit
PROCESSOR AMD Athlon X2 7750z (2.7GHz, 1066MHz, 3MB) edit
OPERATING SYSTEM Microsoft Windows® XP Professional SP3 (Vista 32-bit Business Downgrade to XP (English)) edit
OFFICE SOFTWARE No Productivity Software Requested edit
Dell Recommends
Office Home & Student.
Lets you create great-looking documents, spreadsheets and presentations quickly.
Upgrade to Microsoft® Office Home and Student 2007 [add $149 $99 or $3/month1]
WARRANTY AND SERVICE 1Yr Ltd Hardware Warranty, InHome Service after Remote Diagnosis edit
MEMORY 4GB Dual Channel DDR2 SDRAM at 800MHz- 4 DIMMs edit
HARD DRIVE 500GB Serial ATA Hard Drive (7200RPM) w/DataBurst Cache™ edit
MONITOR 17" Dell E1709W Widescreen Monitor edit
OPTICAL DRIVE 16X DVD+/-RW Drive edit
VIDEO CARD ATI Radeon HD 4350 512MB edit
SOUND Integrated 5.1 Channel Audio edit
SPEAKERS No Speaker Option edit
KEYBOARD & MOUSE Dell Consumer Entry USB Keyboard and Mouse edit
FLOPPY & MEDIA READER Dell 19 in 1 Media Card Reader edit
MODEM 56K PCI Data Fax Modem edit
My Accessories
SECURITY SOFTWARE No Security edit
DATASAFE ONLINE BACKUP Dell Online Backup 2GB for 1 year edit
My Software & Accessories
ALSO INCLUDED WITH YOUR SYSTEM
Inspiron 546 MT Inspiron 546 Minitower w/ Black Bezel
Mouse Mouse included with Keyboard purchase
Network Card Integrated 10/100 Ethernet
Adobe Software Adobe® Reader 9.0